From e8e7e07ffc3b51c43bab170f6f2c4776a580694f Mon Sep 17 00:00:00 2001
From: Claudio Schaad <c.schaad@pog.ch>
Date: Mon, 13 Apr 2026 20:00:18 +0200
Subject: [PATCH] feat: add Nextcloud CalDAV calendar sync

Implements a provider-agnostic calendar sync feature with Nextcloud as the
first alternative to Google Calendar.

- Add ICalendarService interface and provider-agnostic API endpoints
- Implement NextcloudCalendarService using CalDAV (Ical.Net)
- Add CalendarEventBuilder to share event/transport logic across providers
- Add Nextcloud sync button to the UI (Index page)
- Centralise TargetFramework, Nullable and ImplicitUsings in Directory.Build.props (net9.0)
- Add .worktrees to .gitignore and Docker/Nextcloud config docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 .gitignore                                    |    3 +
 Client/Pages/Index.razor                      |   12 +-
 Client/Pages/Index.razor.cs                   |  120 +-
 Client/ShiftScheduler.Client.csproj           |    6 -
 Client/wwwroot/icons/F1_PF.png                |  Bin 0 -> 3420 bytes
 DOCKER_CONFIG.md                              |   16 +-
 Directory.Build.props                         |    7 +
 Server/Controllers/ShiftController.cs         |  122 +-
 Server/Program.cs                             |    1 +
 Server/ShiftScheduler.Server.csproj           |    7 -
 Server/appsettings.json                       |    6 +
 Services/CalendarEventBuilder.cs              |   53 +
 Services/GoogleCalendarService.cs             |  142 +-
 Services/ICalendarService.cs                  |   15 +
 Services/NextcloudCalendarService.cs          |  232 +++
 Services/ShiftScheduler.Services.csproj       |    8 +-
 Shared/CalendarInfo.cs                        |    7 +
 Shared/GoogleCalendar.cs                      |    9 +-
 Shared/ShiftScheduler.Shared.csproj           |    7 +-
 .../CalendarEventBuilderTests.cs              |   69 +
 .../ShiftScheduler.Services.Tests.csproj      |    7 +-
 .../2026-04-10-nextcloud-calendar-sync.md     | 1614 +++++++++++++++++
 ...26-04-10-nextcloud-calendar-sync-design.md |  139 ++
 23 files changed, 2316 insertions(+), 286 deletions(-)
 create mode 100644 Client/wwwroot/icons/F1_PF.png
 create mode 100644 Directory.Build.props
 create mode 100644 Services/CalendarEventBuilder.cs
 create mode 100644 Services/ICalendarService.cs
 create mode 100644 Services/NextcloudCalendarService.cs
 create mode 100644 Shared/CalendarInfo.cs
 create mode 100644 ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs
 create mode 100644 docs/superpowers/plans/2026-04-10-nextcloud-calendar-sync.md
 create mode 100644 docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md

diff --git a/.gitignore b/.gitignore
index a0ae5fc..fcbd2ad 100644
--- a/.gitignore
+++ b/.gitignore
@@ -63,3 +63,6 @@ nunit-*.xml
 # Configuration files (uncomment to avoid committing development configs)
 config/
 **/config/
+
+# Git worktrees
+.worktrees/
diff --git a/Client/Pages/Index.razor b/Client/Pages/Index.razor
index 751dccb..3cdd9e3 100644
--- a/Client/Pages/Index.razor
+++ b/Client/Pages/Index.razor
@@ -9,6 +9,7 @@
     <div class="action-buttons">
         <button class="btn export-btn" @onclick="ExportToIcs" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-calendar" aria-hidden="true"></span> Export to ICS</button>
         <button class="btn export-btn" @onclick="SyncToGoogleCalendar" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-cloud-upload" aria-hidden="true"></span> Sync to Google Calendar</button>
+        <button class="btn export-btn" @onclick="SyncToNextcloud" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-cloud-upload" aria-hidden="true"></span> Sync to Nextcloud</button>
         <button class="btn export-btn" @onclick="ExportToPdf" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-document" aria-hidden="true"></span>Export to PDF</button>
         <button class="btn config-btn" @onclick="ShowConfiguration" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-cog" aria-hidden="true"></span> Configuration</button>
     </div>
@@ -38,7 +39,7 @@
         <div class="modal-dialog modal-dialog-centered">
             <div class="modal-content">
                 <div class="modal-header">
-                    <h5 class="modal-title">Select Google Calendar</h5>
+                    <h5 class="modal-title">Select @(_syncProvider == "nextcloud" ? "Nextcloud" : "Google") Calendar</h5>
                     <button type="button" class="btn-close" @onclick="CloseCalendarSelector"></button>
                 </div>
                 <div class="modal-body">
@@ -87,7 +88,7 @@ else if (_isSyncing)
     <div class="loading-overlay">
         <div class="loading-content">
             <div class="loading-spinner">⏳</div>
-            <p>Syncing with google calendar...</p>
+            <p>Syncing with @(_syncProvider == "nextcloud" ? "Nextcloud" : "Google Calendar")...</p>
         </div>
     </div>
 }
@@ -218,10 +219,3 @@ else
                      OnClose="HideConfiguration" 
                      OnConfigurationChanged="OnConfigurationChanged" />
 
-@code {
-    // Helper method to check if a specific day is loading transport data
-    private bool IsLoadingTransportForDay(DateTime day)
-    {
-        return _isLoadingTransportPerDay.GetValueOrDefault(day, false);
-    }
-}
diff --git a/Client/Pages/Index.razor.cs b/Client/Pages/Index.razor.cs
index 8ba616f..eec64ea 100644
--- a/Client/Pages/Index.razor.cs
+++ b/Client/Pages/Index.razor.cs
@@ -10,7 +10,7 @@ namespace ShiftScheduler.Client.Pages
         [Inject] private HttpClient HttpClient { get; set; } = default!;
         [Inject] private IJSRuntime JSRuntime { get; set; } = default!;
         [Inject] private NavigationManager NavigationManager { get; set; } = default!;
-        
+
         private List<Shift> Shifts { get; set; } = new();
         private Dictionary<DateTime, string> SelectedSchedule { get; set; } = new();
         private Dictionary<DateTime, ShiftWithTransport> SelectedShiftsWithTransport { get; set; } = new();
@@ -21,24 +21,22 @@ namespace ShiftScheduler.Client.Pages
         private bool _showConfigDialog = false;
         private bool _isSyncing = false;
         private bool _showCalendarSelector = false;
+        private string _syncProvider = string.Empty;
         private string _errorMessage = string.Empty;
         private string _successMessage = string.Empty;
-        private List<GoogleCalendar> _availableCalendars = new();
+        private List<CalendarInfo> _availableCalendars = new();
 
         private MonthAndYear SelectedDate => _isCurrentMonth ? MonthAndYear.Current() : MonthAndYear.Next();
 
         protected override async Task OnInitializedAsync()
         {
             _isLoadingInitial = true;
-            
-            // Load shifts first to show the calendar immediately
+
             Shifts = await HttpClient.GetFromJsonAsync<List<Shift>>("api/shift/shifts") ?? new();
-            
-            // Show calendar immediately, don't wait for transport data
+
             _isLoadingInitial = false;
             StateHasChanged();
-            
-            // Load schedule and transport data in the background
+
             _ = Task.Run(async () =>
             {
                 await LoadScheduleFromStorage();
@@ -63,29 +61,25 @@ namespace ShiftScheduler.Client.Pages
         private async Task SelectShift(DateTime day, string shiftName)
         {
             SelectedSchedule[day] = shiftName;
-            
-            // Get transport information for this shift and date - per day loading
+
             _isLoadingTransportPerDay[day] = true;
             StateHasChanged();
-            
+
             try
             {
                 var request = new { ShiftName = shiftName, Date = day };
                 var response = await HttpClient.PostAsJsonAsync("api/shift/shift_transport", request);
-                
+
                 if (response.IsSuccessStatusCode)
                 {
                     var shiftWithTransport = await response.Content.ReadFromJsonAsync<ShiftWithTransport>();
                     if (shiftWithTransport != null)
-                    {
                         SelectedShiftsWithTransport[day] = shiftWithTransport;
-                    }
                 }
             }
             catch (Exception ex)
             {
                 Console.WriteLine($"Error fetching transport data: {ex.Message}");
-                // Create a fallback with just the shift data
                 var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
                 if (shift != null)
                 {
@@ -103,7 +97,7 @@ namespace ShiftScheduler.Client.Pages
                 _isLoadingTransportPerDay[day] = false;
                 StateHasChanged();
             }
-            
+
             await SaveScheduleToStorage();
         }
 
@@ -117,7 +111,7 @@ namespace ShiftScheduler.Client.Pages
 
                 var request = new { Year = SelectedDate.Year, Month = SelectedDate.Month };
                 var response = await HttpClient.PostAsJsonAsync("api/shift/export_ics", request);
-                
+
                 if (response.IsSuccessStatusCode)
                 {
                     var bytes = await response.Content.ReadAsByteArrayAsync();
@@ -136,7 +130,7 @@ namespace ShiftScheduler.Client.Pages
             {
                 _errorMessage = $"Error exporting ICS file: {ex.Message}";
             }
-            
+
             StateHasChanged();
         }
 
@@ -150,7 +144,7 @@ namespace ShiftScheduler.Client.Pages
 
                 var request = new { Year = SelectedDate.Year, Month = SelectedDate.Month };
                 var response = await HttpClient.PostAsJsonAsync("api/shift/export_pdf", request);
-                
+
                 if (response.IsSuccessStatusCode)
                 {
                     var bytes = await response.Content.ReadAsByteArrayAsync();
@@ -168,38 +162,39 @@ namespace ShiftScheduler.Client.Pages
             {
                 _errorMessage = $"Error exporting PDF file: {ex.Message}";
             }
-            
+
             StateHasChanged();
         }
 
-        private async Task SyncToGoogleCalendar()
+        private Task SyncToGoogleCalendar() => StartCalendarSync("google");
+        private Task SyncToNextcloud() => StartCalendarSync("nextcloud");
+
+        private async Task StartCalendarSync(string provider)
         {
             try
             {
                 _errorMessage = string.Empty;
                 _successMessage = string.Empty;
+                _syncProvider = provider;
                 _isSyncing = true;
                 StateHasChanged();
 
-                // Get available calendars
-                var response = await HttpClient.GetAsync("api/shift/google_calendars");
+                var response = await HttpClient.GetAsync($"api/shift/calendars?provider={provider}");
                 if (response.IsSuccessStatusCode)
                 {
-                    _availableCalendars = await response.Content.ReadFromJsonAsync<List<GoogleCalendar>>() ?? new();
-                    
+                    _availableCalendars = await response.Content.ReadFromJsonAsync<List<CalendarInfo>>() ?? new();
+
                     if (_availableCalendars.Count == 1)
                     {
-                        // If only one calendar, sync directly
                         await PerformCalendarSync(_availableCalendars[0].Id);
                     }
                     else if (_availableCalendars.Count > 1)
                     {
-                        // Show calendar selector
                         _showCalendarSelector = true;
                     }
                     else
                     {
-                        _errorMessage = "No writable calendars found in your Google account.";
+                        _errorMessage = "No writable calendars found.";
                     }
                 }
                 else
@@ -210,7 +205,7 @@ namespace ShiftScheduler.Client.Pages
             }
             catch (Exception ex)
             {
-                _errorMessage = $"Error accessing Google Calendar: {ex.Message}";
+                _errorMessage = $"Error accessing calendar: {ex.Message}";
             }
             finally
             {
@@ -234,27 +229,27 @@ namespace ShiftScheduler.Client.Pages
 
                 var request = new
                 {
+                    Provider = _syncProvider,
                     CalendarId = calendarId,
                     Year = SelectedDate.Year,
                     Month = SelectedDate.Month
                 };
 
-                var response = await HttpClient.PostAsJsonAsync("api/shift/sync_to_google_calendar", request);
-                
+                var response = await HttpClient.PostAsJsonAsync("api/shift/sync_to_calendar", request);
+
                 if (response.IsSuccessStatusCode)
                 {
-                    var result = await response.Content.ReadFromJsonAsync<dynamic>();
-                    _successMessage = "Shifts synced to Google Calendar successfully!";
+                    _successMessage = "Shifts synced successfully!";
                 }
                 else
                 {
                     var errorContent = await response.Content.ReadAsStringAsync();
-                    _errorMessage = $"Failed to sync to Google Calendar: {response.StatusCode}. {errorContent}";
+                    _errorMessage = $"Failed to sync: {response.StatusCode}. {errorContent}";
                 }
             }
             catch (Exception ex)
             {
-                _errorMessage = $"Error syncing to Google Calendar: {ex.Message}";
+                _errorMessage = $"Error syncing: {ex.Message}";
             }
             finally
             {
@@ -280,7 +275,7 @@ namespace ShiftScheduler.Client.Pages
                     Month = SelectedDate.Month,
                     Schedule = SelectedSchedule
                 };
-                
+
                 await HttpClient.PostAsJsonAsync("api/shift/save_schedule", request);
             }
             catch (Exception)
@@ -294,12 +289,10 @@ namespace ShiftScheduler.Client.Pages
             try
             {
                 var response = await HttpClient.GetAsync($"api/shift/load_schedule/{SelectedDate.Year}/{SelectedDate.Month}");
-                
+
                 if (response.IsSuccessStatusCode)
                 {
                     SelectedSchedule = await response.Content.ReadFromJsonAsync<Dictionary<DateTime, string>>() ?? new();
-                    
-                    // Reload transport data for all selected shifts
                     await ReloadTransportDataForSchedule();
                 }
                 else
@@ -318,41 +311,33 @@ namespace ShiftScheduler.Client.Pages
         private async Task ReloadTransportDataForSchedule()
         {
             SelectedShiftsWithTransport.Clear();
-            
-            // Clear existing per-day loading states
             _isLoadingTransportPerDay.Clear();
-            
-            // Mark all days as loading
+
             foreach (var kvp in SelectedSchedule)
-            {
                 _isLoadingTransportPerDay[kvp.Key] = true;
-            }
+
             StateHasChanged();
 
-            // Create parallel tasks for all transport data requests
             var transportTasks = SelectedSchedule.Select(async kvp =>
             {
                 var day = kvp.Key;
                 var shiftName = kvp.Value;
-                
+
                 try
                 {
                     var request = new { ShiftName = shiftName, Date = day };
                     var response = await HttpClient.PostAsJsonAsync("api/shift/shift_transport", request);
-                    
+
                     if (response.IsSuccessStatusCode)
                     {
                         var shiftWithTransport = await response.Content.ReadFromJsonAsync<ShiftWithTransport>();
                         if (shiftWithTransport != null)
-                        {
                             SelectedShiftsWithTransport[day] = shiftWithTransport;
-                        }
                     }
                 }
                 catch (Exception ex)
                 {
                     Console.WriteLine($"Error reloading transport data for {day}: {ex.Message}");
-                    // Create fallback without transport
                     var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
                     if (shift != null)
                     {
@@ -367,13 +352,11 @@ namespace ShiftScheduler.Client.Pages
                 }
                 finally
                 {
-                    // Mark this day as no longer loading
                     _isLoadingTransportPerDay[day] = false;
                     await InvokeAsync(StateHasChanged);
                 }
             }).ToArray();
 
-            // Wait for all transport data to load
             await Task.WhenAll(transportTasks);
         }
 
@@ -381,7 +364,7 @@ namespace ShiftScheduler.Client.Pages
         {
             SelectedSchedule.Clear();
             SelectedShiftsWithTransport.Clear();
-            
+
             try
             {
                 await HttpClient.DeleteAsync($"api/shift/delete_schedule/{SelectedDate.Year}/{SelectedDate.Month}");
@@ -392,45 +375,42 @@ namespace ShiftScheduler.Client.Pages
             }
         }
 
-        // Helper method to get transport summary for display
         private string GetTransportSummary(DateTime date)
         {
             if (!SelectedShiftsWithTransport.TryGetValue(date, out var shiftWithTransport))
                 return string.Empty;
 
             var summaries = new List<string>();
-            
+
             var morningTransport = shiftWithTransport.GetMorningTransportSummary();
             if (!string.IsNullOrEmpty(morningTransport))
                 summaries.Add(morningTransport);
-                
+
             var afternoonTransport = shiftWithTransport.GetAfternoonTransportSummary();
             if (!string.IsNullOrEmpty(afternoonTransport))
                 summaries.Add(afternoonTransport);
-            
-            if (summaries.Count == 0) { return string.Empty; }
-                
+
+            if (summaries.Count == 0) return string.Empty;
+
             return "🚂 " + string.Join(" | ", summaries);
         }
 
-        // Helper method to get shift times for display
         private string GetShiftTimes(DateTime date)
         {
             if (!SelectedSchedule.TryGetValue(date, out var shiftName))
                 return string.Empty;
-                
+
             var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
-            if (shift == null)
-                return string.Empty;
-                
+            if (shift == null) return string.Empty;
+
             var times = new List<string>();
-            
+
             if (!string.IsNullOrEmpty(shift.MorningTime))
                 times.Add($"🌅 {shift.MorningTime}");
-                
+
             if (!string.IsNullOrEmpty(shift.AfternoonTime))
                 times.Add($"🌆 {shift.AfternoonTime}");
-                
+
             return string.Join(" | ", times);
         }
 
@@ -448,9 +428,11 @@ namespace ShiftScheduler.Client.Pages
 
         private async Task OnConfigurationChanged()
         {
-            // Reload shifts after configuration changes
             Shifts = await HttpClient.GetFromJsonAsync<List<Shift>>("api/shift/shifts") ?? new();
             StateHasChanged();
         }
+
+        private bool IsLoadingTransportForDay(DateTime day) =>
+            _isLoadingTransportPerDay.GetValueOrDefault(day, false);
     }
 }
diff --git a/Client/ShiftScheduler.Client.csproj b/Client/ShiftScheduler.Client.csproj
index edd0de1..ccd019e 100644
--- a/Client/ShiftScheduler.Client.csproj
+++ b/Client/ShiftScheduler.Client.csproj
@@ -1,11 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">
 
-  <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <ImplicitUsings>enable</ImplicitUsings>
-  </PropertyGroup>
-
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly" Version="9.0.8" />
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Authentication" Version="9.0.8" />
diff --git a/Client/wwwroot/icons/F1_PF.png b/Client/wwwroot/icons/F1_PF.png
new file mode 100644
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diff --git a/DOCKER_CONFIG.md b/DOCKER_CONFIG.md
index 967b82f..55a178e 100644
--- a/DOCKER_CONFIG.md
+++ b/DOCKER_CONFIG.md
@@ -61,4 +61,18 @@ No migration needed - existing installations will automatically:
 
 ## File Format
 
-Configuration files use standard JSON format and can be edited directly if needed. Changes take effect after application restart.
\ No newline at end of file
+Configuration files use standard JSON format and can be edited directly if needed. Changes take effect after application restart.
+
+## Nextcloud Calendar Sync
+
+To enable Nextcloud CalDAV sync, populate the `Nextcloud` section in the production `appsettings.json` (not committed to git):
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "your-nextcloud-username",
+  "AppPassword": "your-nextcloud-app-password"
+}
+```
+
+Generate an app password in Nextcloud: **Settings → Security → Devices & sessions → Create new app password**.
\ No newline at end of file
diff --git a/Directory.Build.props b/Directory.Build.props
new file mode 100644
index 0000000..826b3d4
--- /dev/null
+++ b/Directory.Build.props
@@ -0,0 +1,7 @@
+<Project>
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+</PropertyGroup>
+</Project>
diff --git a/Server/Controllers/ShiftController.cs b/Server/Controllers/ShiftController.cs
index b22ad76..3ce3c89 100644
--- a/Server/Controllers/ShiftController.cs
+++ b/Server/Controllers/ShiftController.cs
@@ -15,19 +15,22 @@ namespace ShiftScheduler.Server.Controllers
         private readonly ITransportService _transportService;
         private readonly IConfigurationService _configurationService;
         private readonly IGoogleCalendarService _googleCalendarService;
+        private readonly INextcloudCalendarService _nextcloudCalendarService;
 
         public ShiftController(
-            IcsExportService icsService, 
-            PdfExportService pdfExportService, 
+            IcsExportService icsService,
+            PdfExportService pdfExportService,
             ITransportService transportService,
             IConfigurationService configurationService,
-            IGoogleCalendarService googleCalendarService)
+            IGoogleCalendarService googleCalendarService,
+            INextcloudCalendarService nextcloudCalendarService)
         {
             _icsService = icsService;
             _pdfExportService = pdfExportService;
             _transportService = transportService;
             _configurationService = configurationService;
             _googleCalendarService = googleCalendarService;
+            _nextcloudCalendarService = nextcloudCalendarService;
         }
 
         [HttpGet("shifts")]
@@ -41,59 +44,38 @@ namespace ShiftScheduler.Server.Controllers
         {
             var shift = _configurationService.GetShifts().FirstOrDefault(s => s.Name == request.ShiftName);
             if (shift == null)
-            {
                 return NotFound($"Shift '{request.ShiftName}' not found");
-            }
 
             var transportConfig = _configurationService.GetTransportConfiguration();
             TransportConnection? morningTransport = null;
             TransportConnection? afternoonTransport = null;
-            
+
             var shifTimes = _configurationService.ParseShiftTimes(request.Date, shift);
 
-            // Get transport for morning shift if it has morning time
-            if (!string.IsNullOrEmpty(shift.MorningTime))
-            {
-                if (shifTimes.MorningStart.HasValue)
-                {
-                    morningTransport = await _transportService.GetConnectionAsync(shifTimes.MorningStart.Value);
-                }
-            }
+            if (!string.IsNullOrEmpty(shift.MorningTime) && shifTimes.MorningStart.HasValue)
+                morningTransport = await _transportService.GetConnectionAsync(shifTimes.MorningStart.Value);
 
-            // Get transport for afternoon shift if it has afternoon time
-            // But only if the break between morning and afternoon is long enough
             if (!string.IsNullOrEmpty(shift.AfternoonTime))
             {
-                var shouldLoadAfternoonTransport = true;
-                
-                // If both morning and afternoon shifts exist, check break duration
-                if (!string.IsNullOrEmpty(shift.MorningTime) && !string.IsNullOrEmpty(shift.AfternoonTime))
+                var shouldLoad = true;
+
+                if (!string.IsNullOrEmpty(shift.MorningTime) && shifTimes.MorningEnd.HasValue && shifTimes.AfternoonStart.HasValue)
                 {
-                    if (shifTimes.MorningEnd.HasValue && shifTimes.AfternoonStart.HasValue)
-                    {
-                        var breakDurationMinutes = (shifTimes.AfternoonStart.Value - shifTimes.MorningEnd.Value).TotalMinutes;
-                        shouldLoadAfternoonTransport = breakDurationMinutes >= transportConfig.MinBreakMinutes;
-                    }
-                }
-                
-                if (shouldLoadAfternoonTransport)
-                {
-                    if (shifTimes.AfternoonStart.HasValue)
-                    {
-                        afternoonTransport = await _transportService.GetConnectionAsync(shifTimes.AfternoonStart.Value);
-                    }
+                    var breakMinutes = (shifTimes.AfternoonStart.Value - shifTimes.MorningEnd.Value).TotalMinutes;
+                    shouldLoad = breakMinutes >= transportConfig.MinBreakMinutes;
                 }
+
+                if (shouldLoad && shifTimes.AfternoonStart.HasValue)
+                    afternoonTransport = await _transportService.GetConnectionAsync(shifTimes.AfternoonStart.Value);
             }
 
-            var shiftWithTransport = new ShiftWithTransport
+            return Ok(new ShiftWithTransport
             {
                 Date = request.Date,
                 Shift = shift,
                 MorningTransport = morningTransport,
-                AfternoonTransport = afternoonTransport,
-            };
-
-            return Ok(shiftWithTransport);
+                AfternoonTransport = afternoonTransport
+            });
         }
 
         [HttpPost("export_ics")]
@@ -154,13 +136,22 @@ namespace ShiftScheduler.Server.Controllers
             }
         }
 
-        [HttpGet("google_calendars")]
-        public async Task<IActionResult> GetGoogleCalendars()
+        [HttpGet("calendars")]
+        public async Task<IActionResult> GetCalendars([FromQuery] string provider)
         {
             try
             {
-                var calendars = await _googleCalendarService.GetCalendarsAsync();
-                return Ok(calendars.Select(c => new { Id = c.Id, Summary = c.Summary }));
+                ICalendarService service = provider switch
+                {
+                    "google"    => (ICalendarService)_googleCalendarService,
+                    "nextcloud" => _nextcloudCalendarService,
+                    _           => null!
+                };
+                if (service is null)
+                    return BadRequest($"Unknown provider '{provider}'. Supported values: google, nextcloud.");
+
+                var calendars = await service.GetCalendarsAsync();
+                return Ok(calendars);
             }
             catch (Exception ex)
             {
@@ -168,18 +159,28 @@ namespace ShiftScheduler.Server.Controllers
             }
         }
 
-        [HttpPost("sync_to_google_calendar")]
-        public async Task<IActionResult> SyncToGoogleCalendar([FromBody] SyncToGoogleCalendarRequest request)
+        [HttpPost("sync_to_calendar")]
+        public async Task<IActionResult> SyncToCalendar([FromBody] SyncToCalendarRequest request)
         {
             try
             {
                 var shiftsWithTransport = await BuildShiftsWithTransportAsync(request.Year, request.Month);
-                await _googleCalendarService.SyncShiftsToCalendarAsync(request.CalendarId, shiftsWithTransport);
-                return Ok(new { message = "Shifts synced to Google Calendar successfully!" });
+
+                ICalendarService service = request.Provider switch
+                {
+                    "google"    => (ICalendarService)_googleCalendarService,
+                    "nextcloud" => _nextcloudCalendarService,
+                    _           => null!
+                };
+                if (service is null)
+                    return BadRequest($"Unknown provider '{request.Provider}'. Supported values: google, nextcloud.");
+
+                await service.SyncShiftsToCalendarAsync(request.CalendarId, shiftsWithTransport);
+                return Ok(new { message = "Shifts synced successfully!" });
             }
             catch (Exception ex)
             {
-                return StatusCode(500, $"Error syncing to Google Calendar: {ex.Message}");
+                return StatusCode(500, $"Error syncing to calendar: {ex.Message}");
             }
         }
 
@@ -193,37 +194,29 @@ namespace ShiftScheduler.Server.Controllers
             {
                 var date = kvp.Key;
                 var shiftName = kvp.Value;
-                
+
                 var shift = _configurationService.GetShifts().FirstOrDefault(s => s.Name == shiftName);
-                if (shift == null)
-                    continue;
+                if (shift == null) continue;
 
                 var shiftTimes = _configurationService.ParseShiftTimes(date, shift);
                 TransportConnection? morningTransport = null;
                 TransportConnection? afternoonTransport = null;
 
-                // Get transport for morning shift if it has morning time
                 if (!string.IsNullOrEmpty(shift.MorningTime) && shiftTimes.MorningStart.HasValue)
-                {
                     morningTransport = await _transportService.GetConnectionAsync(shiftTimes.MorningStart.Value);
-                }
 
-                // Get transport for afternoon shift if it has afternoon time
                 if (!string.IsNullOrEmpty(shift.AfternoonTime))
                 {
-                    var shouldLoadAfternoonTransport = true;
-                    
-                    // If both morning and afternoon shifts exist, check break duration
+                    var shouldLoad = true;
+
                     if (!string.IsNullOrEmpty(shift.MorningTime) && shiftTimes.MorningEnd.HasValue && shiftTimes.AfternoonStart.HasValue)
                     {
-                        var breakDurationMinutes = (shiftTimes.AfternoonStart.Value - shiftTimes.MorningEnd.Value).TotalMinutes;
-                        shouldLoadAfternoonTransport = breakDurationMinutes >= transportConfig.MinBreakMinutes;
+                        var breakMinutes = (shiftTimes.AfternoonStart.Value - shiftTimes.MorningEnd.Value).TotalMinutes;
+                        shouldLoad = breakMinutes >= transportConfig.MinBreakMinutes;
                     }
-                    
-                    if (shouldLoadAfternoonTransport && shiftTimes.AfternoonStart.HasValue)
-                    {
+
+                    if (shouldLoad && shiftTimes.AfternoonStart.HasValue)
                         afternoonTransport = await _transportService.GetConnectionAsync(shiftTimes.AfternoonStart.Value);
-                    }
                 }
 
                 shiftsWithTransport.Add(new ShiftWithTransport
@@ -258,8 +251,9 @@ namespace ShiftScheduler.Server.Controllers
         public int Month { get; set; }
     }
 
-    public class SyncToGoogleCalendarRequest
+    public class SyncToCalendarRequest
     {
+        public string Provider { get; set; } = string.Empty;
         public string CalendarId { get; set; } = string.Empty;
         public int Year { get; set; }
         public int Month { get; set; }
diff --git a/Server/Program.cs b/Server/Program.cs
index 6293348..4a7e574 100644
--- a/Server/Program.cs
+++ b/Server/Program.cs
@@ -30,6 +30,7 @@ builder.Services.AddSingleton<ITransportApiService, TransportApiService>();
 builder.Services.AddSingleton<ITransportService, TransportService>();
 builder.Services.AddHttpContextAccessor();
 builder.Services.AddScoped<IGoogleCalendarService, GoogleCalendarService>();
+builder.Services.AddScoped<INextcloudCalendarService, NextcloudCalendarService>();
 
 // Configure authentication
 builder.Services.AddAuthentication(options =>
diff --git a/Server/ShiftScheduler.Server.csproj b/Server/ShiftScheduler.Server.csproj
index 9dd91d8..62dca14 100644
--- a/Server/ShiftScheduler.Server.csproj
+++ b/Server/ShiftScheduler.Server.csproj
@@ -1,11 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
-  <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <ImplicitUsings>enable</ImplicitUsings>
-  </PropertyGroup>
-
   <ItemGroup>
     <PackageReference Include="Microsoft.AspNetCore.Authentication.Google" Version="9.0.8" />
     <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.Server" Version="9.0.8" />
@@ -17,5 +11,4 @@
     <ProjectReference Include="..\Services\ShiftScheduler.Services.csproj" />
   </ItemGroup>
 
-
 </Project>
diff --git a/Server/appsettings.json b/Server/appsettings.json
index 5c8be59..047fa47 100644
--- a/Server/appsettings.json
+++ b/Server/appsettings.json
@@ -18,6 +18,12 @@
     ]
   },
 
+  "Nextcloud": {
+    "BaseUrl": "https://your-nextcloud-instance.example.com",
+    "Username": "YOUR_NEXTCLOUD_USERNAME",
+    "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+  },
+
   "Transport": {
     "StartStation": "Zurich",
     "EndStation": "Basel",
diff --git a/Services/CalendarEventBuilder.cs b/Services/CalendarEventBuilder.cs
new file mode 100644
index 0000000..1559827
--- /dev/null
+++ b/Services/CalendarEventBuilder.cs
@@ -0,0 +1,53 @@
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public record CalendarEventData(string Summary, string Description);
+
+public static class CalendarEventBuilder
+{
+    public static CalendarEventData BuildEventContent(Shift shift, TransportConnection? transport)
+    {
+        var description = transport != null
+            ? $"Transport: {FormatTransportInfo(transport)}"
+            : string.Empty;
+        return new CalendarEventData(shift.Name, description);
+    }
+
+    public static string FormatTransportInfo(TransportConnection transport)
+    {
+        var departure = transport.DepartureTime.ToString("HH:mm");
+        var arrival = transport.ArrivalTime.ToString("HH:mm");
+        return $"{transport.Platform}: {departure} → {arrival} ({(int)transport.Duration.TotalMinutes} Minutes)";
+    }
+
+    public static async Task<T> ExecuteWithRetryAsync<T>(Func<Task<T>> operation, int maxRetries = 3)
+    {
+        var delay = TimeSpan.FromSeconds(1);
+
+        for (var attempt = 0; attempt <= maxRetries; attempt++)
+        {
+            try
+            {
+                return await operation();
+            }
+            catch (HttpRequestException ex) when (ex.Message.Contains("429") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Rate Limit") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Forbidden") && ex.Message.Contains("quota") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+        }
+
+        return await operation();
+    }
+}
diff --git a/Services/GoogleCalendarService.cs b/Services/GoogleCalendarService.cs
index 4fc9312..0a3f17a 100644
--- a/Services/GoogleCalendarService.cs
+++ b/Services/GoogleCalendarService.cs
@@ -8,41 +8,32 @@ using ShiftScheduler.Shared;
 
 namespace ShiftScheduler.Services;
 
-public interface IGoogleCalendarService
-{
-    Task<List<CalendarListEntry>> GetCalendarsAsync();
-    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
-}
-
 public class GoogleCalendarService(IHttpContextAccessor httpContextAccessor, IConfigurationService configurationService)
     : IGoogleCalendarService
 {
-    public async Task<List<CalendarListEntry>> GetCalendarsAsync()
+    public async Task<List<CalendarInfo>> GetCalendarsAsync()
     {
         var service = await CreateCalendarServiceAsync();
         var request = service.CalendarList.List();
-        var response = await ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
-        
-        return response.Items?.Where(c => c.AccessRole == "owner" || c.AccessRole == "writer").ToList() ?? [];
+        var response = await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
+
+        return response.Items?
+            .Where(c => c.AccessRole == "owner" || c.AccessRole == "writer")
+            .Select(c => new CalendarInfo { Id = c.Id, Summary = c.Summary })
+            .ToList() ?? [];
     }
 
     public async Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts)
     {
         var service = await CreateCalendarServiceAsync();
-        
-        // Get date range for deletion
+
         if (shifts.Count == 0) return;
-        
+
         var minDate = shifts.Min(s => s.Date).Date;
         var maxDate = shifts.Max(s => s.Date).Date.AddDays(1);
-        
-        // Delete existing shift events in the date range
+
         await DeleteExistingShiftEventsAsync(service, calendarId, minDate, maxDate);
-        
-        // Add a small delay to avoid rate limiting
         await Task.Delay(500);
-        
-        // Create new events for each shift with rate limiting protection
         await CreateShiftEventsWithRateLimitingAsync(service, calendarId, shifts);
     }
 
@@ -53,24 +44,23 @@ public class GoogleCalendarService(IHttpContextAccessor httpContextAccessor, ICo
         request.TimeMaxDateTimeOffset = endDate;
         request.SingleEvents = true;
         request.OrderBy = EventsResource.ListRequest.OrderByEnum.StartTime;
-        
-        var response = await ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
-        
+
+        var response = await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
+
         if (response.Items != null)
         {
-            var shiftsToDelete = response.Items.Where(e => 
-                e.ExtendedProperties?.Private__?.ContainsKey("shiftSchedulerEvent") == true).ToList();
-            
-            // Delete events with rate limiting protection
+            var shiftsToDelete = response.Items
+                .Where(e => e.ExtendedProperties?.Private__?.ContainsKey("shiftSchedulerEvent") == true)
+                .ToList();
+
             foreach (var eventToDelete in shiftsToDelete)
             {
-                await ExecuteWithRetryAsync(async () => 
+                await CalendarEventBuilder.ExecuteWithRetryAsync(async () =>
                 {
                     await service.Events.Delete(calendarId, eventToDelete.Id).ExecuteAsync();
-                    return true; // Return something for the generic method
+                    return true;
                 });
-                
-                // Small delay between deletions to avoid rate limiting
+
                 if (shiftsToDelete.Count > 1)
                     await Task.Delay(200);
             }
@@ -82,8 +72,7 @@ public class GoogleCalendarService(IHttpContextAccessor httpContextAccessor, ICo
         foreach (var shiftWithTransport in shifts)
         {
             await CreateShiftEventsAsync(service, calendarId, shiftWithTransport);
-            
-            // Add delay between shift processing to avoid rate limiting
+
             if (shifts.Count > 1)
                 await Task.Delay(300);
         }
@@ -94,118 +83,59 @@ public class GoogleCalendarService(IHttpContextAccessor httpContextAccessor, ICo
         var shift = shiftWithTransport.Shift;
         var date = shiftWithTransport.Date;
 
-        // Skip shifts with no time information
         if (string.IsNullOrEmpty(shift.MorningTime) && string.IsNullOrEmpty(shift.AfternoonTime))
             return;
 
-        // Create morning event if it exists
         if (!string.IsNullOrEmpty(shift.MorningTime))
         {
             var (startTime, endTime) = configurationService.GetZurichTime(DateOnly.FromDateTime(date), shift.MorningTime);
-            var morningEvent = CreateEventFromShift(shift, startTime, endTime, shiftWithTransport.MorningTransport);
-            await ExecuteWithRetryAsync(async () => await service.Events.Insert(morningEvent, calendarId).ExecuteAsync());
+            var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.MorningTransport);
+            var googleEvent = CreateGoogleEvent(eventData, startTime, endTime);
+            await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await service.Events.Insert(googleEvent, calendarId).ExecuteAsync());
         }
 
-        // Create afternoon event if it exists
         if (!string.IsNullOrEmpty(shift.AfternoonTime))
         {
             var (startTime, endTime) = configurationService.GetZurichTime(DateOnly.FromDateTime(date), shift.AfternoonTime);
-            var afternoonEvent = CreateEventFromShift(shift, startTime, endTime, shiftWithTransport.AfternoonTransport);
-            await ExecuteWithRetryAsync(async () => await service.Events.Insert(afternoonEvent, calendarId).ExecuteAsync());
+            var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.AfternoonTransport);
+            var googleEvent = CreateGoogleEvent(eventData, startTime, endTime);
+            await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await service.Events.Insert(googleEvent, calendarId).ExecuteAsync());
         }
     }
 
-    private Event CreateEventFromShift(Shift shift, DateTimeOffset startTime, DateTimeOffset endTime, TransportConnection? transport)
+    private static Event CreateGoogleEvent(CalendarEventData eventData, DateTimeOffset startTime, DateTimeOffset endTime)
     {
-        var summary = $"{shift.Name}";
-        var description = "";
-
-        if (transport != null)
-        {
-            var transportInfo = FormatTransportInfo(transport);
-            description = $"Transport: {transportInfo}";
-        }
-
         return new Event
         {
-            Summary = summary,
-            Description = description,
-            Start = new EventDateTime
-            {
-                DateTimeDateTimeOffset = startTime
-            },
-            End = new EventDateTime
-            {
-                DateTimeDateTimeOffset = endTime
-            },
+            Summary = eventData.Summary,
+            Description = eventData.Description,
+            Start = new EventDateTime { DateTimeDateTimeOffset = startTime },
+            End = new EventDateTime { DateTimeDateTimeOffset = endTime },
             ExtendedProperties = new Event.ExtendedPropertiesData
             {
                 Private__ = new Dictionary<string, string>
                 {
                     ["shiftSchedulerEvent"] = "true",
-                    ["shiftName"] = shift.Name
+                    ["shiftName"] = eventData.Summary
                 }
             }
         };
     }
 
-    private static string FormatTransportInfo(TransportConnection transport)
-    {
-        var departure = transport.DepartureTime.ToString("HH:mm");
-        var arrival = transport.ArrivalTime.ToString("HH:mm");
-        return $"{transport.Platform}: {departure} → {arrival} ({transport.Duration.TotalMinutes} Minutes)";
-    }
-
     private async Task<CalendarService> CreateCalendarServiceAsync()
     {
         var httpContext = httpContextAccessor.HttpContext ?? throw new InvalidOperationException("No HTTP context available");
-        
+
         var accessToken = await httpContext.GetTokenAsync("access_token");
         if (string.IsNullOrEmpty(accessToken))
-        {
             throw new UnauthorizedAccessException("No access token available");
-        }
 
         var credential = GoogleCredential.FromAccessToken(accessToken);
-        
+
         return new CalendarService(new BaseClientService.Initializer
         {
             HttpClientInitializer = credential,
             ApplicationName = "ShiftScheduler"
         });
     }
-
-    private static async Task<T> ExecuteWithRetryAsync<T>(Func<Task<T>> operation, int maxRetries = 3)
-    {
-        var delay = TimeSpan.FromSeconds(1);
-        
-        for (var attempt = 0; attempt <= maxRetries; attempt++)
-        {
-            try
-            {
-                return await operation();
-            }
-            catch (HttpRequestException ex) when (ex.Message.Contains("429") && attempt < maxRetries)
-            {
-                // Rate limit exceeded (HTTP 429), wait with exponential backoff
-                await Task.Delay(delay);
-                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2); // Exponential backoff
-            }
-            catch (Exception ex) when (ex.Message.Contains("Rate Limit") && attempt < maxRetries)
-            {
-                // Another form of rate limit error
-                await Task.Delay(delay);
-                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2); // Exponential backoff
-            }
-            catch (Exception ex) when (ex.Message.Contains("Forbidden") && ex.Message.Contains("quota") && attempt < maxRetries)
-            {
-                // Quota exceeded error
-                await Task.Delay(delay);
-                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2); // Exponential backoff
-            }
-        }
-        
-        // If we get here, all retries failed, execute one more time to get the actual exception
-        return await operation();
-    }
-}
\ No newline at end of file
+}
diff --git a/Services/ICalendarService.cs b/Services/ICalendarService.cs
new file mode 100644
index 0000000..d5ce3ce
--- /dev/null
+++ b/Services/ICalendarService.cs
@@ -0,0 +1,15 @@
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public interface ICalendarService
+{
+    Task<List<CalendarInfo>> GetCalendarsAsync();
+    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
+}
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface IGoogleCalendarService : ICalendarService { }
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface INextcloudCalendarService : ICalendarService { }
diff --git a/Services/NextcloudCalendarService.cs b/Services/NextcloudCalendarService.cs
new file mode 100644
index 0000000..06fb715
--- /dev/null
+++ b/Services/NextcloudCalendarService.cs
@@ -0,0 +1,232 @@
+using Ical.Net;
+using Ical.Net.CalendarComponents;
+using Ical.Net.DataTypes;
+using Ical.Net.Serialization;
+using Microsoft.Extensions.Configuration;
+using ShiftScheduler.Shared;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Xml.Linq;
+
+namespace ShiftScheduler.Services;
+
+public class NextcloudCalendarService(
+    IConfiguration configuration,
+    IHttpClientFactory httpClientFactory,
+    IConfigurationService configurationService) : INextcloudCalendarService
+{
+    private readonly string _baseUrl = configuration["Nextcloud:BaseUrl"]
+        ?? throw new InvalidOperationException("Nextcloud:BaseUrl not configured");
+    private readonly string _username = configuration["Nextcloud:Username"]
+        ?? throw new InvalidOperationException("Nextcloud:Username not configured");
+    private readonly string _appPassword = configuration["Nextcloud:AppPassword"]
+        ?? throw new InvalidOperationException("Nextcloud:AppPassword not configured");
+
+    private static readonly XNamespace DavNs = "DAV:";
+    private static readonly XNamespace CalDavNs = "urn:ietf:params:xml:ns:caldav";
+
+    public async Task<List<CalendarInfo>> GetCalendarsAsync()
+    {
+        var path = $"/remote.php/dav/calendars/{_username}/";
+        var body = """
+            <?xml version="1.0" encoding="utf-8" ?>
+            <D:propfind xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+              <D:prop>
+                <D:displayname/>
+                <D:resourcetype/>
+              </D:prop>
+            </D:propfind>
+            """;
+
+        var request = CreateRequest(HttpMethod.Parse("PROPFIND"), path);
+        request.Headers.Add("Depth", "1");
+        request.Content = new StringContent(body, Encoding.UTF8, "application/xml");
+
+        var client = httpClientFactory.CreateClient();
+        var response = await client.SendAsync(request);
+        response.EnsureSuccessStatusCode();
+
+        var xml = await response.Content.ReadAsStringAsync();
+        return ParseCalendarsFromPropfind(xml);
+    }
+
+    public async Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts)
+    {
+        if (shifts.Count == 0) return;
+
+        var minDate = shifts.Min(s => s.Date).Date;
+        var maxDate = shifts.Max(s => s.Date).Date.AddDays(1);
+
+        await DeleteExistingShiftEventsAsync(calendarId, minDate, maxDate);
+        await Task.Delay(500);
+        await CreateShiftEventsAsync(calendarId, shifts);
+    }
+
+    private List<CalendarInfo> ParseCalendarsFromPropfind(string xml)
+    {
+        var doc = XDocument.Parse(xml);
+        var calendars = new List<CalendarInfo>();
+
+        foreach (var responseElement in doc.Descendants(DavNs + "response"))
+        {
+            var resourceType = responseElement.Descendants(DavNs + "resourcetype").FirstOrDefault();
+            if (resourceType?.Element(CalDavNs + "calendar") == null)
+                continue;
+
+            var href = responseElement.Element(DavNs + "href")?.Value ?? string.Empty;
+            if (string.IsNullOrEmpty(href)) continue;
+
+            var displayName = responseElement.Descendants(DavNs + "displayname").FirstOrDefault()?.Value ?? href;
+
+            calendars.Add(new CalendarInfo { Id = href, Summary = displayName });
+        }
+
+        return calendars;
+    }
+
+    private async Task DeleteExistingShiftEventsAsync(string calendarId, DateTime startDate, DateTime endDate)
+    {
+        var startUtc = startDate.ToUniversalTime().ToString("yyyyMMddTHHmmssZ");
+        var endUtc = endDate.ToUniversalTime().ToString("yyyyMMddTHHmmssZ");
+
+        var body = $"""
+            <?xml version="1.0" encoding="utf-8" ?>
+            <C:calendar-query xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+              <D:prop>
+                <D:getetag/>
+                <C:calendar-data/>
+              </D:prop>
+              <C:filter>
+                <C:comp-filter name="VCALENDAR">
+                  <C:comp-filter name="VEVENT">
+                    <C:time-range start="{startUtc}" end="{endUtc}"/>
+                  </C:comp-filter>
+                </C:comp-filter>
+              </C:filter>
+            </C:calendar-query>
+            """;
+
+        var reportRequest = CreateRequest(HttpMethod.Parse("REPORT"), calendarId);
+        reportRequest.Headers.Add("Depth", "1");
+        reportRequest.Content = new StringContent(body, Encoding.UTF8, "application/xml");
+
+        var client = httpClientFactory.CreateClient();
+        var reportResponse = await client.SendAsync(reportRequest);
+        reportResponse.EnsureSuccessStatusCode();
+
+        var xml = await reportResponse.Content.ReadAsStringAsync();
+        var eventUrls = ParseShiftEventUrls(xml);
+
+        foreach (var url in eventUrls)
+        {
+            var deleteRequest = CreateRequest(HttpMethod.Delete, url);
+            var deleteResponse = await client.SendAsync(deleteRequest);
+
+            if (!deleteResponse.IsSuccessStatusCode &&
+                deleteResponse.StatusCode != System.Net.HttpStatusCode.NotFound)
+                deleteResponse.EnsureSuccessStatusCode();
+
+            if (eventUrls.Count > 1)
+                await Task.Delay(200);
+        }
+    }
+
+    private List<string> ParseShiftEventUrls(string xml)
+    {
+        var doc = XDocument.Parse(xml);
+        var urls = new List<string>();
+
+        foreach (var responseElement in doc.Descendants(DavNs + "response"))
+        {
+            var calendarData = responseElement
+                .Descendants(CalDavNs + "calendar-data")
+                .FirstOrDefault()?.Value;
+
+            if (calendarData == null || !calendarData.Contains("X-SHIFT-SCHEDULER:TRUE"))
+                continue;
+
+            var href = responseElement.Element(DavNs + "href")?.Value;
+            if (!string.IsNullOrEmpty(href))
+                urls.Add(href);
+        }
+
+        return urls;
+    }
+
+    private async Task CreateShiftEventsAsync(string calendarId, List<ShiftWithTransport> shifts)
+    {
+        var client = httpClientFactory.CreateClient();
+
+        foreach (var shiftWithTransport in shifts)
+        {
+            var shift = shiftWithTransport.Shift;
+            var date = shiftWithTransport.Date;
+
+            if (string.IsNullOrEmpty(shift.MorningTime) && string.IsNullOrEmpty(shift.AfternoonTime))
+                continue;
+
+            if (!string.IsNullOrEmpty(shift.MorningTime))
+            {
+                var (startTime, endTime) = configurationService.GetZurichTime(
+                    DateOnly.FromDateTime(date), shift.MorningTime);
+                var uid = $"shift-{SanitizeForUid(shift.Name)}-{date:yyyyMMdd}-morning";
+                var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.MorningTransport);
+                var icsContent = BuildIcsContent(uid, eventData, startTime, endTime);
+
+                var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
+                putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
+                (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
+            }
+
+            if (!string.IsNullOrEmpty(shift.AfternoonTime))
+            {
+                var (startTime, endTime) = configurationService.GetZurichTime(
+                    DateOnly.FromDateTime(date), shift.AfternoonTime);
+                var uid = $"shift-{SanitizeForUid(shift.Name)}-{date:yyyyMMdd}-afternoon";
+                var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.AfternoonTransport);
+                var icsContent = BuildIcsContent(uid, eventData, startTime, endTime);
+
+                var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
+                putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
+                (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
+            }
+
+            if (shifts.Count > 1)
+                await Task.Delay(300);
+        }
+    }
+
+    private static string BuildIcsContent(string uid, CalendarEventData eventData, DateTimeOffset startTime, DateTimeOffset endTime)
+    {
+        var calendar = new Calendar();
+        var vevent = new CalendarEvent
+        {
+            Uid = uid,
+            Summary = eventData.Summary,
+            Description = eventData.Description,
+            DtStart = new CalDateTime(startTime.UtcDateTime, hasTime: true),
+            DtEnd = new CalDateTime(endTime.UtcDateTime, hasTime: true)
+        };
+        vevent.Properties.Add(new Ical.Net.CalendarProperty("X-SHIFT-SCHEDULER", "TRUE"));
+        calendar.Events.Add(vevent);
+        return new CalendarSerializer().SerializeToString(calendar)!;
+    }
+
+    private static string SanitizeForUid(string name)
+    {
+        return new string(name.Select(c => char.IsLetterOrDigit(c) ? c : '-').ToArray());
+    }
+
+    private HttpRequestMessage CreateRequest(HttpMethod method, string path)
+    {
+        // path may be a URL-path from a CalDAV href ("/remote.php/...") or a full URL
+        var uri = path.StartsWith("http", StringComparison.OrdinalIgnoreCase)
+            ? new Uri(path)
+            : new Uri($"{_baseUrl.TrimEnd('/')}{path}");
+
+        var request = new HttpRequestMessage(method, uri);
+        var credentials = Convert.ToBase64String(Encoding.UTF8.GetBytes($"{_username}:{_appPassword}"));
+        request.Headers.Authorization = new AuthenticationHeaderValue("Basic", credentials);
+        return request;
+    }
+}
diff --git a/Services/ShiftScheduler.Services.csproj b/Services/ShiftScheduler.Services.csproj
index a4e95c2..2ddd5ef 100644
--- a/Services/ShiftScheduler.Services.csproj
+++ b/Services/ShiftScheduler.Services.csproj
@@ -1,11 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
-  </PropertyGroup>
-
   <ItemGroup>
     <ProjectReference Include="..\Shared\ShiftScheduler.Shared.csproj" />
   </ItemGroup>
@@ -15,6 +9,8 @@
     <PackageReference Include="Ical.Net" Version="5.1.0" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.Abstractions" Version="2.3.0" />
     <PackageReference Include="Microsoft.AspNetCore.Http.Abstractions" Version="2.3.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="10.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Http" Version="10.0.5" />
     <PackageReference Include="QuestPDF" Version="2025.7.0" />
     <PackageReference Include="Microsoft.Extensions.Caching.Memory" Version="9.0.0" />
   </ItemGroup>
diff --git a/Shared/CalendarInfo.cs b/Shared/CalendarInfo.cs
new file mode 100644
index 0000000..a7df061
--- /dev/null
+++ b/Shared/CalendarInfo.cs
@@ -0,0 +1,7 @@
+namespace ShiftScheduler.Shared;
+
+public class CalendarInfo
+{
+    public string Id { get; set; } = string.Empty;
+    public string Summary { get; set; } = string.Empty;
+}
diff --git a/Shared/GoogleCalendar.cs b/Shared/GoogleCalendar.cs
index 81ca4e0..b8318b6 100644
--- a/Shared/GoogleCalendar.cs
+++ b/Shared/GoogleCalendar.cs
@@ -1,7 +1,2 @@
-namespace ShiftScheduler.Shared;
-
-public class GoogleCalendar
-{
-    public string Id { get; set; } = string.Empty;
-    public string Summary { get; set; } = string.Empty;
-}
\ No newline at end of file
+// Class renamed to CalendarInfo — see Shared/CalendarInfo.cs
+namespace ShiftScheduler.Shared;
\ No newline at end of file
diff --git a/Shared/ShiftScheduler.Shared.csproj b/Shared/ShiftScheduler.Shared.csproj
index 46f7f7d..63a9106 100644
--- a/Shared/ShiftScheduler.Shared.csproj
+++ b/Shared/ShiftScheduler.Shared.csproj
@@ -1,12 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
-  <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <ImplicitUsings>enable</ImplicitUsings>
-  </PropertyGroup>
-
   <ItemGroup>
     <SupportedPlatform Include="browser" />
   </ItemGroup>
+
 </Project>
diff --git a/ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs b/ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs
new file mode 100644
index 0000000..62852c5
--- /dev/null
+++ b/ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs
@@ -0,0 +1,69 @@
+using ShiftScheduler.Services;
+using ShiftScheduler.Shared;
+using Shouldly;
+
+namespace ShiftScheduler.Services.Tests;
+
+public class CalendarEventBuilderTests
+{
+    [Fact]
+    public void BuildEventContent_WithTransport_IncludesTransportInDescription()
+    {
+        var shift = new Shift { Name = "Früh" };
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, transport);
+
+        result.Summary.ShouldBe("Früh");
+        result.Description.ShouldBe("Transport: Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void BuildEventContent_WithoutTransport_EmptyDescription()
+    {
+        var shift = new Shift { Name = "Spät" };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, null);
+
+        result.Summary.ShouldBe("Spät");
+        result.Description.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void FormatTransportInfo_FormatsCorrectly()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe("Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void FormatTransportInfo_WithNullPlatform_UsesEmptyString()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = null,
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe(": 05:30 → 05:55 (25 Minutes)");
+    }
+}
diff --git a/ShiftScheduler.Services.Tests/ShiftScheduler.Services.Tests.csproj b/ShiftScheduler.Services.Tests/ShiftScheduler.Services.Tests.csproj
index feb6af5..651049c 100644
--- a/ShiftScheduler.Services.Tests/ShiftScheduler.Services.Tests.csproj
+++ b/ShiftScheduler.Services.Tests/ShiftScheduler.Services.Tests.csproj
@@ -1,9 +1,6 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net9.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
-    <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 
@@ -15,7 +12,7 @@
     <PackageReference Include="xunit" Version="2.9.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
   </ItemGroup>
-  
+
   <ItemGroup>
     <ProjectReference Include="..\Services\ShiftScheduler.Services.csproj" />
     <ProjectReference Include="..\Shared\ShiftScheduler.Shared.csproj" />
diff --git a/docs/superpowers/plans/2026-04-10-nextcloud-calendar-sync.md b/docs/superpowers/plans/2026-04-10-nextcloud-calendar-sync.md
new file mode 100644
index 0000000..b7eca7c
--- /dev/null
+++ b/docs/superpowers/plans/2026-04-10-nextcloud-calendar-sync.md
@@ -0,0 +1,1614 @@
+# Nextcloud Calendar Sync Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Add Nextcloud CalDAV as a second calendar sync target alongside Google Calendar, with shared base logic extracted into a common interface and static helper.
+
+**Architecture:** A new `ICalendarService` interface unifies both providers. A static `CalendarEventBuilder` holds shared pure logic (event content building, transport formatting, retry). `GoogleCalendarService` and `NextcloudCalendarService` each implement a marker interface extending `ICalendarService`. The controller routes to the right service based on a `provider` query/body parameter.
+
+**Tech Stack:** .NET 9, ASP.NET Core, Blazor WASM, Google.Apis.Calendar.v3 (existing), Ical.Net (existing), CalDAV over HttpClient, System.Xml.Linq for CalDAV XML parsing.
+
+**Design spec:** `docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md`
+
+---
+
+## File Map
+
+| Action | File | Responsibility |
+|--------|------|----------------|
+| Create | `Shared/CalendarInfo.cs` | Shared DTO replacing `GoogleCalendar` |
+| Modify | `Shared/GoogleCalendar.cs` | Empty out (class renamed to `CalendarInfo`) |
+| Create | `Services/ICalendarService.cs` | Shared interface + marker interfaces |
+| Create | `Services/CalendarEventBuilder.cs` | Pure helpers: event content, transport format, retry |
+| Modify | `Services/GoogleCalendarService.cs` | Implement `IGoogleCalendarService`, use `CalendarEventBuilder` |
+| Create | `Services/NextcloudCalendarService.cs` | CalDAV implementation of `INextcloudCalendarService` |
+| Modify | `Server/appsettings.json` | Add `Nextcloud` config section with dummy values |
+| Modify | `Server/Program.cs` | Register `NextcloudCalendarService`, `IHttpClientFactory` already present |
+| Modify | `Server/Controllers/ShiftController.cs` | New provider-agnostic endpoints |
+| Modify | `Client/Pages/Index.razor.cs` | Nextcloud sync flow, rename `GoogleCalendar` → `CalendarInfo` |
+| Modify | `Client/Pages/Index.razor` | Add Nextcloud button, dynamic modal title |
+| Modify | `DOCKER_CONFIG.md` | Document Nextcloud config section |
+| Create | `ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs` | Unit tests for `CalendarEventBuilder` |
+
+---
+
+## Task 1: Add `CalendarInfo` shared DTO
+
+**Files:**
+- Create: `Shared/CalendarInfo.cs`
+- Modify: `Shared/GoogleCalendar.cs`
+
+- [ ] **Step 1: Create `Shared/CalendarInfo.cs`**
+
+```csharp
+namespace ShiftScheduler.Shared;
+
+public class CalendarInfo
+{
+    public string Id { get; set; } = string.Empty;
+    public string Summary { get; set; } = string.Empty;
+}
+```
+
+- [ ] **Step 2: Empty out `Shared/GoogleCalendar.cs`**
+
+Replace entire file content with:
+
+```csharp
+// Class renamed to CalendarInfo — see Shared/CalendarInfo.cs
+namespace ShiftScheduler.Shared;
+```
+
+- [ ] **Step 3: Verify build**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build
+```
+
+Expected: Build succeeds (no code uses `GoogleCalendar` yet — references are updated in later tasks).
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add Shared/CalendarInfo.cs Shared/GoogleCalendar.cs
+git commit -m "feat: add CalendarInfo shared DTO, retire GoogleCalendar"
+```
+
+---
+
+## Task 2: Create `ICalendarService` interface
+
+**Files:**
+- Create: `Services/ICalendarService.cs`
+
+- [ ] **Step 1: Create `Services/ICalendarService.cs`**
+
+```csharp
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public interface ICalendarService
+{
+    Task<List<CalendarInfo>> GetCalendarsAsync();
+    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
+}
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface IGoogleCalendarService : ICalendarService { }
+
+/// <summary>Marker interface for DI disambiguation.</summary>
+public interface INextcloudCalendarService : ICalendarService { }
+```
+
+- [ ] **Step 2: Verify build**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build
+```
+
+Expected: Build succeeds.
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add Services/ICalendarService.cs
+git commit -m "feat: add ICalendarService interface and marker interfaces"
+```
+
+---
+
+## Task 3: Create `CalendarEventBuilder` with tests
+
+**Files:**
+- Create: `ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs`
+- Create: `Services/CalendarEventBuilder.cs`
+
+- [ ] **Step 1: Write failing tests**
+
+Create `ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs`:
+
+```csharp
+using ShiftScheduler.Services;
+using ShiftScheduler.Shared;
+using Shouldly;
+
+namespace ShiftScheduler.Services.Tests;
+
+public class CalendarEventBuilderTests
+{
+    [Fact]
+    public void BuildEventContent_WithTransport_IncludesTransportInDescription()
+    {
+        var shift = new Shift { Name = "Früh" };
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, transport);
+
+        result.Summary.ShouldBe("Früh");
+        result.Description.ShouldBe("Transport: Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void BuildEventContent_WithoutTransport_EmptyDescription()
+    {
+        var shift = new Shift { Name = "Spät" };
+
+        var result = CalendarEventBuilder.BuildEventContent(shift, null);
+
+        result.Summary.ShouldBe("Spät");
+        result.Description.ShouldBeEmpty();
+    }
+
+    [Fact]
+    public void FormatTransportInfo_FormatsCorrectly()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = "Gleis 3",
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe("Gleis 3: 05:30 → 05:55 (25 Minutes)");
+    }
+
+    [Fact]
+    public void FormatTransportInfo_WithNullPlatform_UsesEmptyString()
+    {
+        var transport = new TransportConnection
+        {
+            Platform = null,
+            DepartureTime = DateTimeOffset.Parse("2026-04-01T05:30:00+02:00"),
+            ArrivalTime = DateTimeOffset.Parse("2026-04-01T05:55:00+02:00"),
+            Duration = TimeSpan.FromMinutes(25)
+        };
+
+        var result = CalendarEventBuilder.FormatTransportInfo(transport);
+
+        result.ShouldBe(": 05:30 → 05:55 (25 Minutes)");
+    }
+}
+```
+
+- [ ] **Step 2: Run tests to confirm they fail**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet test ShiftScheduler.Services.Tests/ --filter "CalendarEventBuilderTests" -v minimal
+```
+
+Expected: Build error — `CalendarEventBuilder` does not exist yet.
+
+- [ ] **Step 3: Create `Services/CalendarEventBuilder.cs`**
+
+```csharp
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public record CalendarEventData(string Summary, string Description);
+
+public static class CalendarEventBuilder
+{
+    public static CalendarEventData BuildEventContent(Shift shift, TransportConnection? transport)
+    {
+        var description = transport != null
+            ? $"Transport: {FormatTransportInfo(transport)}"
+            : string.Empty;
+        return new CalendarEventData(shift.Name, description);
+    }
+
+    public static string FormatTransportInfo(TransportConnection transport)
+    {
+        var departure = transport.DepartureTime.ToString("HH:mm");
+        var arrival = transport.ArrivalTime.ToString("HH:mm");
+        return $"{transport.Platform}: {departure} → {arrival} ({transport.Duration.TotalMinutes} Minutes)";
+    }
+
+    public static async Task<T> ExecuteWithRetryAsync<T>(Func<Task<T>> operation, int maxRetries = 3)
+    {
+        var delay = TimeSpan.FromSeconds(1);
+
+        for (var attempt = 0; attempt <= maxRetries; attempt++)
+        {
+            try
+            {
+                return await operation();
+            }
+            catch (HttpRequestException ex) when (ex.Message.Contains("429") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Rate Limit") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+            catch (Exception ex) when (ex.Message.Contains("Forbidden") && ex.Message.Contains("quota") && attempt < maxRetries)
+            {
+                await Task.Delay(delay);
+                delay = TimeSpan.FromMilliseconds(delay.TotalMilliseconds * 2);
+            }
+        }
+
+        return await operation();
+    }
+}
+```
+
+- [ ] **Step 4: Run tests to confirm they pass**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet test ShiftScheduler.Services.Tests/ --filter "CalendarEventBuilderTests" -v minimal
+```
+
+Expected: 4 tests pass.
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add Services/CalendarEventBuilder.cs ShiftScheduler.Services.Tests/CalendarEventBuilderTests.cs
+git commit -m "feat: add CalendarEventBuilder with shared event/transport logic"
+```
+
+---
+
+## Task 4: Refactor `GoogleCalendarService`
+
+**Files:**
+- Modify: `Services/GoogleCalendarService.cs`
+
+The refactoring:
+1. Change implemented interface from `IGoogleCalendarService` (old, deleted) to the new `IGoogleCalendarService : ICalendarService`
+2. Change `GetCalendarsAsync()` return type from `List<CalendarListEntry>` to `List<CalendarInfo>`
+3. Replace `FormatTransportInfo` and the summary/description building with `CalendarEventBuilder`
+4. Replace the private `ExecuteWithRetryAsync` with `CalendarEventBuilder.ExecuteWithRetryAsync`
+
+- [ ] **Step 1: Replace `Services/GoogleCalendarService.cs`**
+
+```csharp
+using Google.Apis.Auth.OAuth2;
+using Google.Apis.Calendar.v3;
+using Google.Apis.Calendar.v3.Data;
+using Google.Apis.Services;
+using Microsoft.AspNetCore.Authentication;
+using Microsoft.AspNetCore.Http;
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Services;
+
+public class GoogleCalendarService(IHttpContextAccessor httpContextAccessor, IConfigurationService configurationService)
+    : IGoogleCalendarService
+{
+    public async Task<List<CalendarInfo>> GetCalendarsAsync()
+    {
+        var service = await CreateCalendarServiceAsync();
+        var request = service.CalendarList.List();
+        var response = await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
+
+        return response.Items?
+            .Where(c => c.AccessRole == "owner" || c.AccessRole == "writer")
+            .Select(c => new CalendarInfo { Id = c.Id, Summary = c.Summary })
+            .ToList() ?? [];
+    }
+
+    public async Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts)
+    {
+        var service = await CreateCalendarServiceAsync();
+
+        if (shifts.Count == 0) return;
+
+        var minDate = shifts.Min(s => s.Date).Date;
+        var maxDate = shifts.Max(s => s.Date).Date.AddDays(1);
+
+        await DeleteExistingShiftEventsAsync(service, calendarId, minDate, maxDate);
+        await Task.Delay(500);
+        await CreateShiftEventsWithRateLimitingAsync(service, calendarId, shifts);
+    }
+
+    private async Task DeleteExistingShiftEventsAsync(CalendarService service, string calendarId, DateTime startDate, DateTime endDate)
+    {
+        var request = service.Events.List(calendarId);
+        request.TimeMinDateTimeOffset = startDate;
+        request.TimeMaxDateTimeOffset = endDate;
+        request.SingleEvents = true;
+        request.OrderBy = EventsResource.ListRequest.OrderByEnum.StartTime;
+
+        var response = await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await request.ExecuteAsync());
+
+        if (response.Items != null)
+        {
+            var shiftsToDelete = response.Items
+                .Where(e => e.ExtendedProperties?.Private__?.ContainsKey("shiftSchedulerEvent") == true)
+                .ToList();
+
+            foreach (var eventToDelete in shiftsToDelete)
+            {
+                await CalendarEventBuilder.ExecuteWithRetryAsync(async () =>
+                {
+                    await service.Events.Delete(calendarId, eventToDelete.Id).ExecuteAsync();
+                    return true;
+                });
+
+                if (shiftsToDelete.Count > 1)
+                    await Task.Delay(200);
+            }
+        }
+    }
+
+    private async Task CreateShiftEventsWithRateLimitingAsync(CalendarService service, string calendarId, List<ShiftWithTransport> shifts)
+    {
+        foreach (var shiftWithTransport in shifts)
+        {
+            await CreateShiftEventsAsync(service, calendarId, shiftWithTransport);
+
+            if (shifts.Count > 1)
+                await Task.Delay(300);
+        }
+    }
+
+    private async Task CreateShiftEventsAsync(CalendarService service, string calendarId, ShiftWithTransport shiftWithTransport)
+    {
+        var shift = shiftWithTransport.Shift;
+        var date = shiftWithTransport.Date;
+
+        if (string.IsNullOrEmpty(shift.MorningTime) && string.IsNullOrEmpty(shift.AfternoonTime))
+            return;
+
+        if (!string.IsNullOrEmpty(shift.MorningTime))
+        {
+            var (startTime, endTime) = configurationService.GetZurichTime(DateOnly.FromDateTime(date), shift.MorningTime);
+            var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.MorningTransport);
+            var googleEvent = CreateGoogleEvent(eventData, startTime, endTime);
+            await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await service.Events.Insert(googleEvent, calendarId).ExecuteAsync());
+        }
+
+        if (!string.IsNullOrEmpty(shift.AfternoonTime))
+        {
+            var (startTime, endTime) = configurationService.GetZurichTime(DateOnly.FromDateTime(date), shift.AfternoonTime);
+            var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.AfternoonTransport);
+            var googleEvent = CreateGoogleEvent(eventData, startTime, endTime);
+            await CalendarEventBuilder.ExecuteWithRetryAsync(async () => await service.Events.Insert(googleEvent, calendarId).ExecuteAsync());
+        }
+    }
+
+    private static Event CreateGoogleEvent(CalendarEventData eventData, DateTimeOffset startTime, DateTimeOffset endTime)
+    {
+        return new Event
+        {
+            Summary = eventData.Summary,
+            Description = eventData.Description,
+            Start = new EventDateTime { DateTimeDateTimeOffset = startTime },
+            End = new EventDateTime { DateTimeDateTimeOffset = endTime },
+            ExtendedProperties = new Event.ExtendedPropertiesData
+            {
+                Private__ = new Dictionary<string, string>
+                {
+                    ["shiftSchedulerEvent"] = "true",
+                    ["shiftName"] = eventData.Summary
+                }
+            }
+        };
+    }
+
+    private async Task<CalendarService> CreateCalendarServiceAsync()
+    {
+        var httpContext = httpContextAccessor.HttpContext ?? throw new InvalidOperationException("No HTTP context available");
+
+        var accessToken = await httpContext.GetTokenAsync("access_token");
+        if (string.IsNullOrEmpty(accessToken))
+            throw new UnauthorizedAccessException("No access token available");
+
+        var credential = GoogleCredential.FromAccessToken(accessToken);
+
+        return new CalendarService(new BaseClientService.Initializer
+        {
+            HttpClientInitializer = credential,
+            ApplicationName = "ShiftScheduler"
+        });
+    }
+}
+```
+
+- [ ] **Step 2: Build and run all tests**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build && dotnet test ShiftScheduler.Services.Tests/ -v minimal
+```
+
+Expected: Build succeeds, all tests pass.
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add Services/GoogleCalendarService.cs
+git commit -m "refactor: GoogleCalendarService implements ICalendarService, uses CalendarEventBuilder"
+```
+
+---
+
+## Task 5: Add Nextcloud configuration
+
+**Files:**
+- Modify: `Server/appsettings.json`
+- Modify: `DOCKER_CONFIG.md`
+
+- [ ] **Step 1: Add Nextcloud section to `Server/appsettings.json`**
+
+Add after the `"Authentication"` block:
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "YOUR_NEXTCLOUD_USERNAME",
+  "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+},
+```
+
+The full `Authentication` + `Nextcloud` section in `appsettings.json` should look like:
+
+```json
+"Authentication": {
+  "Google": {
+    "ClientId": "YOUR_GOOGLE_CLIENT_ID",
+    "ClientSecret": "YOUR_GOOGLE_CLIENT_SECRET"
+  },
+  "AuthorizedEmails": [
+    "example1@gmail.com",
+    "example2@gmail.com"
+  ]
+},
+
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "YOUR_NEXTCLOUD_USERNAME",
+  "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+},
+```
+
+- [ ] **Step 2: Update `DOCKER_CONFIG.md`**
+
+Add a new section at the end of the file:
+
+```markdown
+## Nextcloud Calendar Sync
+
+To enable Nextcloud CalDAV sync, populate the `Nextcloud` section in the production `appsettings.json` (not committed to git):
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "your-nextcloud-username",
+  "AppPassword": "your-nextcloud-app-password"
+}
+```
+
+Generate an app password in Nextcloud: **Settings → Security → Devices & sessions → Create new app password**.
+```
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add Server/appsettings.json DOCKER_CONFIG.md
+git commit -m "feat: add Nextcloud config placeholder and Docker docs"
+```
+
+---
+
+## Task 6: Implement `NextcloudCalendarService`
+
+**Files:**
+- Create: `Services/NextcloudCalendarService.cs`
+
+- [ ] **Step 1: Create `Services/NextcloudCalendarService.cs`**
+
+```csharp
+using Ical.Net;
+using Ical.Net.CalendarComponents;
+using Ical.Net.DataTypes;
+using Ical.Net.Serialization;
+using Microsoft.Extensions.Configuration;
+using ShiftScheduler.Shared;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Xml.Linq;
+
+namespace ShiftScheduler.Services;
+
+public class NextcloudCalendarService(
+    IConfiguration configuration,
+    IHttpClientFactory httpClientFactory,
+    IConfigurationService configurationService) : INextcloudCalendarService
+{
+    private readonly string _baseUrl = configuration["Nextcloud:BaseUrl"]
+        ?? throw new InvalidOperationException("Nextcloud:BaseUrl not configured");
+    private readonly string _username = configuration["Nextcloud:Username"]
+        ?? throw new InvalidOperationException("Nextcloud:Username not configured");
+    private readonly string _appPassword = configuration["Nextcloud:AppPassword"]
+        ?? throw new InvalidOperationException("Nextcloud:AppPassword not configured");
+
+    private static readonly XNamespace DavNs = "DAV:";
+    private static readonly XNamespace CalDavNs = "urn:ietf:params:xml:ns:caldav";
+
+    public async Task<List<CalendarInfo>> GetCalendarsAsync()
+    {
+        var path = $"/remote.php/dav/calendars/{_username}/";
+        var body = """
+            <?xml version="1.0" encoding="utf-8" ?>
+            <D:propfind xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+              <D:prop>
+                <D:displayname/>
+                <D:resourcetype/>
+              </D:prop>
+            </D:propfind>
+            """;
+
+        var request = CreateRequest(HttpMethod.Parse("PROPFIND"), path);
+        request.Headers.Add("Depth", "1");
+        request.Content = new StringContent(body, Encoding.UTF8, "application/xml");
+
+        var client = httpClientFactory.CreateClient();
+        var response = await client.SendAsync(request);
+        response.EnsureSuccessStatusCode();
+
+        var xml = await response.Content.ReadAsStringAsync();
+        return ParseCalendarsFromPropfind(xml);
+    }
+
+    public async Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts)
+    {
+        if (shifts.Count == 0) return;
+
+        var minDate = shifts.Min(s => s.Date).Date;
+        var maxDate = shifts.Max(s => s.Date).Date.AddDays(1);
+
+        await DeleteExistingShiftEventsAsync(calendarId, minDate, maxDate);
+        await Task.Delay(500);
+        await CreateShiftEventsAsync(calendarId, shifts);
+    }
+
+    private List<CalendarInfo> ParseCalendarsFromPropfind(string xml)
+    {
+        var doc = XDocument.Parse(xml);
+        var calendars = new List<CalendarInfo>();
+
+        foreach (var responseElement in doc.Descendants(DavNs + "response"))
+        {
+            var resourceType = responseElement.Descendants(DavNs + "resourcetype").FirstOrDefault();
+            if (resourceType?.Element(CalDavNs + "calendar") == null)
+                continue;
+
+            var href = responseElement.Element(DavNs + "href")?.Value ?? string.Empty;
+            if (string.IsNullOrEmpty(href)) continue;
+
+            var displayName = responseElement.Descendants(DavNs + "displayname").FirstOrDefault()?.Value ?? href;
+
+            calendars.Add(new CalendarInfo { Id = href, Summary = displayName });
+        }
+
+        return calendars;
+    }
+
+    private async Task DeleteExistingShiftEventsAsync(string calendarId, DateTime startDate, DateTime endDate)
+    {
+        var startUtc = startDate.ToUniversalTime().ToString("yyyyMMddTHHmmssZ");
+        var endUtc = endDate.ToUniversalTime().ToString("yyyyMMddTHHmmssZ");
+
+        var body = $"""
+            <?xml version="1.0" encoding="utf-8" ?>
+            <C:calendar-query xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
+              <D:prop>
+                <D:getetag/>
+                <C:calendar-data/>
+              </D:prop>
+              <C:filter>
+                <C:comp-filter name="VCALENDAR">
+                  <C:comp-filter name="VEVENT">
+                    <C:time-range start="{startUtc}" end="{endUtc}"/>
+                  </C:comp-filter>
+                </C:comp-filter>
+              </C:filter>
+            </C:calendar-query>
+            """;
+
+        var reportRequest = CreateRequest(HttpMethod.Parse("REPORT"), calendarId);
+        reportRequest.Headers.Add("Depth", "1");
+        reportRequest.Content = new StringContent(body, Encoding.UTF8, "application/xml");
+
+        var client = httpClientFactory.CreateClient();
+        var reportResponse = await client.SendAsync(reportRequest);
+        reportResponse.EnsureSuccessStatusCode();
+
+        var xml = await reportResponse.Content.ReadAsStringAsync();
+        var eventUrls = ParseShiftEventUrls(xml);
+
+        foreach (var url in eventUrls)
+        {
+            var deleteRequest = CreateRequest(HttpMethod.Delete, url);
+            var deleteResponse = await client.SendAsync(deleteRequest);
+
+            if (!deleteResponse.IsSuccessStatusCode &&
+                deleteResponse.StatusCode != System.Net.HttpStatusCode.NotFound)
+                deleteResponse.EnsureSuccessStatusCode();
+
+            if (eventUrls.Count > 1)
+                await Task.Delay(200);
+        }
+    }
+
+    private List<string> ParseShiftEventUrls(string xml)
+    {
+        var doc = XDocument.Parse(xml);
+        var urls = new List<string>();
+
+        foreach (var responseElement in doc.Descendants(DavNs + "response"))
+        {
+            var calendarData = responseElement
+                .Descendants(CalDavNs + "calendar-data")
+                .FirstOrDefault()?.Value;
+
+            if (calendarData == null || !calendarData.Contains("X-SHIFT-SCHEDULER:TRUE"))
+                continue;
+
+            var href = responseElement.Element(DavNs + "href")?.Value;
+            if (!string.IsNullOrEmpty(href))
+                urls.Add(href);
+        }
+
+        return urls;
+    }
+
+    private async Task CreateShiftEventsAsync(string calendarId, List<ShiftWithTransport> shifts)
+    {
+        var client = httpClientFactory.CreateClient();
+
+        foreach (var shiftWithTransport in shifts)
+        {
+            var shift = shiftWithTransport.Shift;
+            var date = shiftWithTransport.Date;
+
+            if (string.IsNullOrEmpty(shift.MorningTime) && string.IsNullOrEmpty(shift.AfternoonTime))
+                continue;
+
+            if (!string.IsNullOrEmpty(shift.MorningTime))
+            {
+                var (startTime, endTime) = configurationService.GetZurichTime(
+                    DateOnly.FromDateTime(date), shift.MorningTime);
+                var uid = $"shift-{SanitizeForUid(shift.Name)}-{date:yyyyMMdd}-morning";
+                var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.MorningTransport);
+                var icsContent = BuildIcsContent(uid, eventData, startTime, endTime);
+
+                var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
+                putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
+                (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
+            }
+
+            if (!string.IsNullOrEmpty(shift.AfternoonTime))
+            {
+                var (startTime, endTime) = configurationService.GetZurichTime(
+                    DateOnly.FromDateTime(date), shift.AfternoonTime);
+                var uid = $"shift-{SanitizeForUid(shift.Name)}-{date:yyyyMMdd}-afternoon";
+                var eventData = CalendarEventBuilder.BuildEventContent(shift, shiftWithTransport.AfternoonTransport);
+                var icsContent = BuildIcsContent(uid, eventData, startTime, endTime);
+
+                var putRequest = CreateRequest(HttpMethod.Put, $"{calendarId}{Uri.EscapeDataString(uid)}.ics");
+                putRequest.Content = new StringContent(icsContent, Encoding.UTF8, "text/calendar");
+                (await client.SendAsync(putRequest)).EnsureSuccessStatusCode();
+            }
+
+            if (shifts.Count > 1)
+                await Task.Delay(300);
+        }
+    }
+
+    private static string BuildIcsContent(string uid, CalendarEventData eventData, DateTimeOffset startTime, DateTimeOffset endTime)
+    {
+        var calendar = new Calendar();
+        var vevent = new CalendarEvent
+        {
+            Uid = uid,
+            Summary = eventData.Summary,
+            Description = eventData.Description,
+            DtStart = new CalDateTime(startTime.UtcDateTime) { IsUniversalTime = true },
+            DtEnd = new CalDateTime(endTime.UtcDateTime) { IsUniversalTime = true }
+        };
+        vevent.Properties.Add(new Ical.Net.CalendarProperty("X-SHIFT-SCHEDULER", "TRUE"));
+        calendar.Events.Add(vevent);
+        return new CalendarSerializer().SerializeToString(calendar);
+    }
+
+    private static string SanitizeForUid(string name)
+    {
+        return new string(name.Select(c => char.IsLetterOrDigit(c) ? c : '-').ToArray());
+    }
+
+    private HttpRequestMessage CreateRequest(HttpMethod method, string path)
+    {
+        // path may be a URL-path from a CalDAV href ("/remote.php/...") or a full URL
+        var uri = path.StartsWith("http", StringComparison.OrdinalIgnoreCase)
+            ? new Uri(path)
+            : new Uri($"{_baseUrl.TrimEnd('/')}{path}");
+
+        var request = new HttpRequestMessage(method, uri);
+        var credentials = Convert.ToBase64String(Encoding.UTF8.GetBytes($"{_username}:{_appPassword}"));
+        request.Headers.Authorization = new AuthenticationHeaderValue("Basic", credentials);
+        return request;
+    }
+}
+```
+
+- [ ] **Step 2: Build**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build
+```
+
+Expected: Build succeeds.
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add Services/NextcloudCalendarService.cs
+git commit -m "feat: implement NextcloudCalendarService with CalDAV"
+```
+
+---
+
+## Task 7: Update `Program.cs` and `ShiftController`
+
+**Files:**
+- Modify: `Server/Program.cs`
+- Modify: `Server/Controllers/ShiftController.cs`
+
+- [ ] **Step 1: Update DI registration in `Server/Program.cs`**
+
+Replace the line:
+```csharp
+builder.Services.AddScoped<IGoogleCalendarService, GoogleCalendarService>();
+```
+
+With:
+```csharp
+builder.Services.AddScoped<IGoogleCalendarService, GoogleCalendarService>();
+builder.Services.AddScoped<INextcloudCalendarService, NextcloudCalendarService>();
+```
+
+Note: `IHttpClientFactory` is already registered by the existing `builder.Services.AddHttpClient<TransportApiService>();` call — no additional registration needed.
+
+- [ ] **Step 2: Update `Server/Controllers/ShiftController.cs`**
+
+Replace the full file with:
+
+```csharp
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using ShiftScheduler.Services;
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Server.Controllers
+{
+    [ApiController]
+    [Route("api/[controller]")]
+    [Authorize(Policy = "AllowedEmails")]
+    public class ShiftController : ControllerBase
+    {
+        private readonly IcsExportService _icsService;
+        private readonly PdfExportService _pdfExportService;
+        private readonly ITransportService _transportService;
+        private readonly IConfigurationService _configurationService;
+        private readonly IGoogleCalendarService _googleCalendarService;
+        private readonly INextcloudCalendarService _nextcloudCalendarService;
+
+        public ShiftController(
+            IcsExportService icsService,
+            PdfExportService pdfExportService,
+            ITransportService transportService,
+            IConfigurationService configurationService,
+            IGoogleCalendarService googleCalendarService,
+            INextcloudCalendarService nextcloudCalendarService)
+        {
+            _icsService = icsService;
+            _pdfExportService = pdfExportService;
+            _transportService = transportService;
+            _configurationService = configurationService;
+            _googleCalendarService = googleCalendarService;
+            _nextcloudCalendarService = nextcloudCalendarService;
+        }
+
+        [HttpGet("shifts")]
+        public IActionResult GetShifts()
+        {
+            return Ok(_configurationService.GetShifts());
+        }
+
+        [HttpPost("shift_transport")]
+        public async Task<IActionResult> GetShiftTransport([FromBody] ShiftTransportRequest request)
+        {
+            var shift = _configurationService.GetShifts().FirstOrDefault(s => s.Name == request.ShiftName);
+            if (shift == null)
+                return NotFound($"Shift '{request.ShiftName}' not found");
+
+            var transportConfig = _configurationService.GetTransportConfiguration();
+            TransportConnection? morningTransport = null;
+            TransportConnection? afternoonTransport = null;
+
+            var shifTimes = _configurationService.ParseShiftTimes(request.Date, shift);
+
+            if (!string.IsNullOrEmpty(shift.MorningTime) && shifTimes.MorningStart.HasValue)
+                morningTransport = await _transportService.GetConnectionAsync(shifTimes.MorningStart.Value);
+
+            if (!string.IsNullOrEmpty(shift.AfternoonTime))
+            {
+                var shouldLoad = true;
+
+                if (!string.IsNullOrEmpty(shift.MorningTime) && shifTimes.MorningEnd.HasValue && shifTimes.AfternoonStart.HasValue)
+                {
+                    var breakMinutes = (shifTimes.AfternoonStart.Value - shifTimes.MorningEnd.Value).TotalMinutes;
+                    shouldLoad = breakMinutes >= transportConfig.MinBreakMinutes;
+                }
+
+                if (shouldLoad && shifTimes.AfternoonStart.HasValue)
+                    afternoonTransport = await _transportService.GetConnectionAsync(shifTimes.AfternoonStart.Value);
+            }
+
+            return Ok(new ShiftWithTransport
+            {
+                Date = request.Date,
+                Shift = shift,
+                MorningTransport = morningTransport,
+                AfternoonTransport = afternoonTransport
+            });
+        }
+
+        [HttpPost("export_ics")]
+        public async Task<IActionResult> ExportIcs([FromBody] ExportRequest request)
+        {
+            var shiftsWithTransport = await BuildShiftsWithTransportAsync(request.Year, request.Month);
+            var ics = _icsService.GenerateIcs(shiftsWithTransport);
+            return File(System.Text.Encoding.UTF8.GetBytes(ics), "text/calendar", "schedule.ics");
+        }
+
+        [HttpPost("export_pdf")]
+        public async Task<IActionResult> ExportPdf([FromBody] ExportRequest request)
+        {
+            var shiftsWithTransport = await BuildShiftsWithTransportAsync(request.Year, request.Month);
+            var pdf = _pdfExportService.GenerateMonthlySchedulePdf(shiftsWithTransport);
+            return File(pdf, "application/pdf", "schedule.pdf");
+        }
+
+        [HttpPost("save_schedule")]
+        public async Task<IActionResult> SaveSchedule([FromBody] SaveScheduleRequest request)
+        {
+            try
+            {
+                await _configurationService.SaveScheduleAsync(request.Year, request.Month, request.Schedule);
+                return Ok();
+            }
+            catch (Exception ex)
+            {
+                return StatusCode(500, $"Error saving schedule: {ex.Message}");
+            }
+        }
+
+        [HttpGet("load_schedule/{year}/{month}")]
+        public async Task<IActionResult> LoadSchedule(int year, int month)
+        {
+            try
+            {
+                var schedule = await _configurationService.LoadScheduleAsync(year, month);
+                return Ok(schedule);
+            }
+            catch (Exception ex)
+            {
+                return StatusCode(500, $"Error loading schedule: {ex.Message}");
+            }
+        }
+
+        [HttpDelete("delete_schedule/{year}/{month}")]
+        public async Task<IActionResult> DeleteSchedule(int year, int month)
+        {
+            try
+            {
+                await _configurationService.DeleteScheduleAsync(year, month);
+                return Ok();
+            }
+            catch (Exception ex)
+            {
+                return StatusCode(500, $"Error deleting schedule: {ex.Message}");
+            }
+        }
+
+        [HttpGet("calendars")]
+        public async Task<IActionResult> GetCalendars([FromQuery] string provider)
+        {
+            try
+            {
+                ICalendarService service = provider == "nextcloud"
+                    ? _nextcloudCalendarService
+                    : _googleCalendarService;
+
+                var calendars = await service.GetCalendarsAsync();
+                return Ok(calendars);
+            }
+            catch (Exception ex)
+            {
+                return StatusCode(500, $"Error retrieving calendars: {ex.Message}");
+            }
+        }
+
+        [HttpPost("sync_to_calendar")]
+        public async Task<IActionResult> SyncToCalendar([FromBody] SyncToCalendarRequest request)
+        {
+            try
+            {
+                var shiftsWithTransport = await BuildShiftsWithTransportAsync(request.Year, request.Month);
+
+                ICalendarService service = request.Provider == "nextcloud"
+                    ? _nextcloudCalendarService
+                    : _googleCalendarService;
+
+                await service.SyncShiftsToCalendarAsync(request.CalendarId, shiftsWithTransport);
+                return Ok(new { message = "Shifts synced successfully!" });
+            }
+            catch (Exception ex)
+            {
+                return StatusCode(500, $"Error syncing to calendar: {ex.Message}");
+            }
+        }
+
+        private async Task<List<ShiftWithTransport>> BuildShiftsWithTransportAsync(int year, int month)
+        {
+            var schedule = await _configurationService.LoadScheduleAsync(year, month);
+            var shiftsWithTransport = new List<ShiftWithTransport>();
+            var transportConfig = _configurationService.GetTransportConfiguration();
+
+            foreach (var kvp in schedule)
+            {
+                var date = kvp.Key;
+                var shiftName = kvp.Value;
+
+                var shift = _configurationService.GetShifts().FirstOrDefault(s => s.Name == shiftName);
+                if (shift == null) continue;
+
+                var shiftTimes = _configurationService.ParseShiftTimes(date, shift);
+                TransportConnection? morningTransport = null;
+                TransportConnection? afternoonTransport = null;
+
+                if (!string.IsNullOrEmpty(shift.MorningTime) && shiftTimes.MorningStart.HasValue)
+                    morningTransport = await _transportService.GetConnectionAsync(shiftTimes.MorningStart.Value);
+
+                if (!string.IsNullOrEmpty(shift.AfternoonTime))
+                {
+                    var shouldLoad = true;
+
+                    if (!string.IsNullOrEmpty(shift.MorningTime) && shiftTimes.MorningEnd.HasValue && shiftTimes.AfternoonStart.HasValue)
+                    {
+                        var breakMinutes = (shiftTimes.AfternoonStart.Value - shiftTimes.MorningEnd.Value).TotalMinutes;
+                        shouldLoad = breakMinutes >= transportConfig.MinBreakMinutes;
+                    }
+
+                    if (shouldLoad && shiftTimes.AfternoonStart.HasValue)
+                        afternoonTransport = await _transportService.GetConnectionAsync(shiftTimes.AfternoonStart.Value);
+                }
+
+                shiftsWithTransport.Add(new ShiftWithTransport
+                {
+                    Date = date,
+                    Shift = shift,
+                    MorningTransport = morningTransport,
+                    AfternoonTransport = afternoonTransport
+                });
+            }
+
+            return shiftsWithTransport;
+        }
+    }
+
+    public class ShiftTransportRequest
+    {
+        public string ShiftName { get; set; } = string.Empty;
+        public DateTime Date { get; set; }
+    }
+
+    public class SaveScheduleRequest
+    {
+        public int Year { get; set; }
+        public int Month { get; set; }
+        public Dictionary<DateTime, string> Schedule { get; set; } = new();
+    }
+
+    public class ExportRequest
+    {
+        public int Year { get; set; }
+        public int Month { get; set; }
+    }
+
+    public class SyncToCalendarRequest
+    {
+        public string Provider { get; set; } = string.Empty;
+        public string CalendarId { get; set; } = string.Empty;
+        public int Year { get; set; }
+        public int Month { get; set; }
+    }
+}
+```
+
+- [ ] **Step 3: Build**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build
+```
+
+Expected: Build succeeds.
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add Server/Program.cs Server/Controllers/ShiftController.cs
+git commit -m "feat: add provider-agnostic calendar endpoints, register NextcloudCalendarService"
+```
+
+---
+
+## Task 8: Update Blazor client
+
+**Files:**
+- Modify: `Client/Pages/Index.razor.cs`
+- Modify: `Client/Pages/Index.razor`
+
+- [ ] **Step 1: Replace `Client/Pages/Index.razor.cs`**
+
+Replace the full file:
+
+```csharp
+using Microsoft.AspNetCore.Components;
+using Microsoft.JSInterop;
+using System.Net.Http.Json;
+using ShiftScheduler.Shared;
+
+namespace ShiftScheduler.Client.Pages
+{
+    public partial class Index : ComponentBase
+    {
+        [Inject] private HttpClient HttpClient { get; set; } = default!;
+        [Inject] private IJSRuntime JSRuntime { get; set; } = default!;
+        [Inject] private NavigationManager NavigationManager { get; set; } = default!;
+
+        private List<Shift> Shifts { get; set; } = new();
+        private Dictionary<DateTime, string> SelectedSchedule { get; set; } = new();
+        private Dictionary<DateTime, ShiftWithTransport> SelectedShiftsWithTransport { get; set; } = new();
+        private Dictionary<DateTime, bool> _isLoadingTransportPerDay { get; set; } = new();
+
+        private bool _isCurrentMonth = true;
+        private bool _isLoadingInitial = false;
+        private bool _showConfigDialog = false;
+        private bool _isSyncing = false;
+        private bool _showCalendarSelector = false;
+        private string _syncProvider = string.Empty;
+        private string _errorMessage = string.Empty;
+        private string _successMessage = string.Empty;
+        private List<CalendarInfo> _availableCalendars = new();
+
+        private MonthAndYear SelectedDate => _isCurrentMonth ? MonthAndYear.Current() : MonthAndYear.Next();
+
+        protected override async Task OnInitializedAsync()
+        {
+            _isLoadingInitial = true;
+
+            Shifts = await HttpClient.GetFromJsonAsync<List<Shift>>("api/shift/shifts") ?? new();
+
+            _isLoadingInitial = false;
+            StateHasChanged();
+
+            _ = Task.Run(async () =>
+            {
+                await LoadScheduleFromStorage();
+                await InvokeAsync(StateHasChanged);
+            });
+        }
+
+        private async Task SelectCurrentMonth()
+        {
+            _isCurrentMonth = true;
+            await LoadScheduleFromStorage();
+            StateHasChanged();
+        }
+
+        private async Task SelectNextMonth()
+        {
+            _isCurrentMonth = false;
+            await LoadScheduleFromStorage();
+            StateHasChanged();
+        }
+
+        private async Task SelectShift(DateTime day, string shiftName)
+        {
+            SelectedSchedule[day] = shiftName;
+
+            _isLoadingTransportPerDay[day] = true;
+            StateHasChanged();
+
+            try
+            {
+                var request = new { ShiftName = shiftName, Date = day };
+                var response = await HttpClient.PostAsJsonAsync("api/shift/shift_transport", request);
+
+                if (response.IsSuccessStatusCode)
+                {
+                    var shiftWithTransport = await response.Content.ReadFromJsonAsync<ShiftWithTransport>();
+                    if (shiftWithTransport != null)
+                        SelectedShiftsWithTransport[day] = shiftWithTransport;
+                }
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine($"Error fetching transport data: {ex.Message}");
+                var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
+                if (shift != null)
+                {
+                    SelectedShiftsWithTransport[day] = new ShiftWithTransport
+                    {
+                        Date = day,
+                        Shift = shift,
+                        MorningTransport = null,
+                        AfternoonTransport = null
+                    };
+                }
+            }
+            finally
+            {
+                _isLoadingTransportPerDay[day] = false;
+                StateHasChanged();
+            }
+
+            await SaveScheduleToStorage();
+        }
+
+        private async Task ExportToIcs()
+        {
+            try
+            {
+                _errorMessage = string.Empty;
+                _successMessage = string.Empty;
+                StateHasChanged();
+
+                var request = new { Year = SelectedDate.Year, Month = SelectedDate.Month };
+                var response = await HttpClient.PostAsJsonAsync("api/shift/export_ics", request);
+
+                if (response.IsSuccessStatusCode)
+                {
+                    var bytes = await response.Content.ReadAsByteArrayAsync();
+                    var base64 = Convert.ToBase64String(bytes);
+                    var fileUrl = $"data:text/calendar;base64,{base64}";
+                    NavigationManager.NavigateTo(fileUrl, true);
+                    _successMessage = "ICS file exported successfully!";
+                }
+                else
+                {
+                    var errorContent = await response.Content.ReadAsStringAsync();
+                    _errorMessage = $"Failed to export ICS file: {response.StatusCode}. {errorContent}";
+                }
+            }
+            catch (Exception ex)
+            {
+                _errorMessage = $"Error exporting ICS file: {ex.Message}";
+            }
+
+            StateHasChanged();
+        }
+
+        private async Task ExportToPdf()
+        {
+            try
+            {
+                _errorMessage = string.Empty;
+                _successMessage = string.Empty;
+                StateHasChanged();
+
+                var request = new { Year = SelectedDate.Year, Month = SelectedDate.Month };
+                var response = await HttpClient.PostAsJsonAsync("api/shift/export_pdf", request);
+
+                if (response.IsSuccessStatusCode)
+                {
+                    var bytes = await response.Content.ReadAsByteArrayAsync();
+                    var base64 = Convert.ToBase64String(bytes);
+                    await JSRuntime.InvokeVoidAsync("downloadFile", $"Schedule {SelectedDate.Year}-{SelectedDate.Month:D2}.pdf", "application/pdf", base64);
+                    _successMessage = "PDF file exported successfully!";
+                }
+                else
+                {
+                    var errorContent = await response.Content.ReadAsStringAsync();
+                    _errorMessage = $"Failed to export PDF file: {response.StatusCode}. {errorContent}";
+                }
+            }
+            catch (Exception ex)
+            {
+                _errorMessage = $"Error exporting PDF file: {ex.Message}";
+            }
+
+            StateHasChanged();
+        }
+
+        private Task SyncToGoogleCalendar() => StartCalendarSync("google");
+        private Task SyncToNextcloud() => StartCalendarSync("nextcloud");
+
+        private async Task StartCalendarSync(string provider)
+        {
+            try
+            {
+                _errorMessage = string.Empty;
+                _successMessage = string.Empty;
+                _syncProvider = provider;
+                _isSyncing = true;
+                StateHasChanged();
+
+                var response = await HttpClient.GetAsync($"api/shift/calendars?provider={provider}");
+                if (response.IsSuccessStatusCode)
+                {
+                    _availableCalendars = await response.Content.ReadFromJsonAsync<List<CalendarInfo>>() ?? new();
+
+                    if (_availableCalendars.Count == 1)
+                    {
+                        await PerformCalendarSync(_availableCalendars[0].Id);
+                    }
+                    else if (_availableCalendars.Count > 1)
+                    {
+                        _showCalendarSelector = true;
+                    }
+                    else
+                    {
+                        _errorMessage = "No writable calendars found.";
+                    }
+                }
+                else
+                {
+                    var errorContent = await response.Content.ReadAsStringAsync();
+                    _errorMessage = $"Failed to retrieve calendars: {response.StatusCode}. {errorContent}";
+                }
+            }
+            catch (Exception ex)
+            {
+                _errorMessage = $"Error accessing calendar: {ex.Message}";
+            }
+            finally
+            {
+                _isSyncing = false;
+                StateHasChanged();
+            }
+        }
+
+        private async Task SelectCalendarAndSync(string calendarId)
+        {
+            _showCalendarSelector = false;
+            await PerformCalendarSync(calendarId);
+        }
+
+        private async Task PerformCalendarSync(string calendarId)
+        {
+            try
+            {
+                _isSyncing = true;
+                StateHasChanged();
+
+                var request = new
+                {
+                    Provider = _syncProvider,
+                    CalendarId = calendarId,
+                    Year = SelectedDate.Year,
+                    Month = SelectedDate.Month
+                };
+
+                var response = await HttpClient.PostAsJsonAsync("api/shift/sync_to_calendar", request);
+
+                if (response.IsSuccessStatusCode)
+                {
+                    _successMessage = "Shifts synced successfully!";
+                }
+                else
+                {
+                    var errorContent = await response.Content.ReadAsStringAsync();
+                    _errorMessage = $"Failed to sync: {response.StatusCode}. {errorContent}";
+                }
+            }
+            catch (Exception ex)
+            {
+                _errorMessage = $"Error syncing: {ex.Message}";
+            }
+            finally
+            {
+                _isSyncing = false;
+                StateHasChanged();
+            }
+        }
+
+        private void CloseCalendarSelector()
+        {
+            _showCalendarSelector = false;
+            _availableCalendars.Clear();
+            StateHasChanged();
+        }
+
+        private async Task SaveScheduleToStorage()
+        {
+            try
+            {
+                var request = new
+                {
+                    Year = SelectedDate.Year,
+                    Month = SelectedDate.Month,
+                    Schedule = SelectedSchedule
+                };
+
+                await HttpClient.PostAsJsonAsync("api/shift/save_schedule", request);
+            }
+            catch (Exception)
+            {
+                // ignored
+            }
+        }
+
+        private async Task LoadScheduleFromStorage()
+        {
+            try
+            {
+                var response = await HttpClient.GetAsync($"api/shift/load_schedule/{SelectedDate.Year}/{SelectedDate.Month}");
+
+                if (response.IsSuccessStatusCode)
+                {
+                    SelectedSchedule = await response.Content.ReadFromJsonAsync<Dictionary<DateTime, string>>() ?? new();
+                    await ReloadTransportDataForSchedule();
+                }
+                else
+                {
+                    SelectedSchedule = new Dictionary<DateTime, string>();
+                    SelectedShiftsWithTransport = new Dictionary<DateTime, ShiftWithTransport>();
+                }
+            }
+            catch (Exception)
+            {
+                SelectedSchedule = new Dictionary<DateTime, string>();
+                SelectedShiftsWithTransport = new Dictionary<DateTime, ShiftWithTransport>();
+            }
+        }
+
+        private async Task ReloadTransportDataForSchedule()
+        {
+            SelectedShiftsWithTransport.Clear();
+            _isLoadingTransportPerDay.Clear();
+
+            foreach (var kvp in SelectedSchedule)
+                _isLoadingTransportPerDay[kvp.Key] = true;
+
+            StateHasChanged();
+
+            var transportTasks = SelectedSchedule.Select(async kvp =>
+            {
+                var day = kvp.Key;
+                var shiftName = kvp.Value;
+
+                try
+                {
+                    var request = new { ShiftName = shiftName, Date = day };
+                    var response = await HttpClient.PostAsJsonAsync("api/shift/shift_transport", request);
+
+                    if (response.IsSuccessStatusCode)
+                    {
+                        var shiftWithTransport = await response.Content.ReadFromJsonAsync<ShiftWithTransport>();
+                        if (shiftWithTransport != null)
+                            SelectedShiftsWithTransport[day] = shiftWithTransport;
+                    }
+                }
+                catch (Exception ex)
+                {
+                    Console.WriteLine($"Error reloading transport data for {day}: {ex.Message}");
+                    var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
+                    if (shift != null)
+                    {
+                        SelectedShiftsWithTransport[day] = new ShiftWithTransport
+                        {
+                            Date = day,
+                            Shift = shift,
+                            MorningTransport = null,
+                            AfternoonTransport = null
+                        };
+                    }
+                }
+                finally
+                {
+                    _isLoadingTransportPerDay[day] = false;
+                    await InvokeAsync(StateHasChanged);
+                }
+            }).ToArray();
+
+            await Task.WhenAll(transportTasks);
+        }
+
+        private async Task ResetSchedule()
+        {
+            SelectedSchedule.Clear();
+            SelectedShiftsWithTransport.Clear();
+
+            try
+            {
+                await HttpClient.DeleteAsync($"api/shift/delete_schedule/{SelectedDate.Year}/{SelectedDate.Month}");
+            }
+            catch (Exception)
+            {
+                // ignored
+            }
+        }
+
+        private string GetTransportSummary(DateTime date)
+        {
+            if (!SelectedShiftsWithTransport.TryGetValue(date, out var shiftWithTransport))
+                return string.Empty;
+
+            var summaries = new List<string>();
+
+            var morningTransport = shiftWithTransport.GetMorningTransportSummary();
+            if (!string.IsNullOrEmpty(morningTransport))
+                summaries.Add(morningTransport);
+
+            var afternoonTransport = shiftWithTransport.GetAfternoonTransportSummary();
+            if (!string.IsNullOrEmpty(afternoonTransport))
+                summaries.Add(afternoonTransport);
+
+            if (summaries.Count == 0) return string.Empty;
+
+            return "🚂 " + string.Join(" | ", summaries);
+        }
+
+        private string GetShiftTimes(DateTime date)
+        {
+            if (!SelectedSchedule.TryGetValue(date, out var shiftName))
+                return string.Empty;
+
+            var shift = Shifts.FirstOrDefault(s => s.Name == shiftName);
+            if (shift == null) return string.Empty;
+
+            var times = new List<string>();
+
+            if (!string.IsNullOrEmpty(shift.MorningTime))
+                times.Add($"🌅 {shift.MorningTime}");
+
+            if (!string.IsNullOrEmpty(shift.AfternoonTime))
+                times.Add($"🌆 {shift.AfternoonTime}");
+
+            return string.Join(" | ", times);
+        }
+
+        private void ShowConfiguration()
+        {
+            _showConfigDialog = true;
+            StateHasChanged();
+        }
+
+        private void HideConfiguration()
+        {
+            _showConfigDialog = false;
+            StateHasChanged();
+        }
+
+        private async Task OnConfigurationChanged()
+        {
+            Shifts = await HttpClient.GetFromJsonAsync<List<Shift>>("api/shift/shifts") ?? new();
+            StateHasChanged();
+        }
+
+        private bool IsLoadingTransportForDay(DateTime day) =>
+            _isLoadingTransportPerDay.GetValueOrDefault(day, false);
+    }
+}
+```
+
+- [ ] **Step 2: Update `Client/Pages/Index.razor`**
+
+Make the following targeted changes to `Index.razor`:
+
+**2a. Add Nextcloud button** — in the `action-buttons` div, after the Google Calendar button, add:
+
+```html
+<button class="btn export-btn" @onclick="SyncToNextcloud" disabled="@(_isLoadingInitial || _isSyncing)"><span class="oi oi-cloud-upload" aria-hidden="true"></span> Sync to Nextcloud</button>
+```
+
+**2b. Update the calendar selector modal title** — change:
+
+```html
+<h5 class="modal-title">Select Google Calendar</h5>
+```
+
+To:
+
+```html
+<h5 class="modal-title">Select @(_syncProvider == "nextcloud" ? "Nextcloud" : "Google") Calendar</h5>
+```
+
+**2d. Remove the `@code` block at the bottom of `Index.razor`** — the `IsLoadingTransportForDay` helper method was moved into `Index.razor.cs`. Delete this block from `Index.razor`:
+
+```razor
+@code {
+    // Helper method to check if a specific day is loading transport data
+    private bool IsLoadingTransportForDay(DateTime day)
+    {
+        return _isLoadingTransportPerDay.GetValueOrDefault(day, false);
+    }
+}
+```
+
+**2e. Update the syncing loading overlay text** — change:
+
+```html
+<p>Syncing with google calendar...</p>
+```
+
+To:
+
+```html
+<p>Syncing with @(_syncProvider == "nextcloud" ? "Nextcloud" : "Google Calendar")...</p>
+```
+
+- [ ] **Step 3: Build**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet build
+```
+
+Expected: Build succeeds.
+
+- [ ] **Step 4: Run all tests**
+
+```bash
+cd /home/schaad/Documents/ShiftScheduler && dotnet test ShiftScheduler.Services.Tests/ -v minimal
+```
+
+Expected: All tests pass.
+
+- [ ] **Step 5: Commit**
+
+```bash
+git add Client/Pages/Index.razor.cs Client/Pages/Index.razor
+git commit -m "feat: add Nextcloud sync button and update UI for provider-agnostic calendar flow"
+```
+
+---
+
+## Verification Checklist
+
+After all tasks are complete, verify end-to-end:
+
+- [ ] `dotnet build` succeeds with no warnings on `GoogleCalendar` references
+- [ ] `dotnet test ShiftScheduler.Services.Tests/` — all tests pass
+- [ ] Both "Sync to Google Calendar" and "Sync to Nextcloud" buttons appear in the UI
+- [ ] Modal title changes based on which button was pressed
+- [ ] `GET /api/shift/calendars?provider=google` and `?provider=nextcloud` both return `CalendarInfo[]`
+- [ ] `POST /api/shift/sync_to_calendar` with `Provider: "google"` still works as before
diff --git a/docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md b/docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md
new file mode 100644
index 0000000..47ca150
--- /dev/null
+++ b/docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md
@@ -0,0 +1,139 @@
+# Nextcloud Calendar Sync — Design Spec
+
+**Date:** 2026-04-10  
+**Status:** Approved
+
+---
+
+## Overview
+
+Add a second calendar sync target (Nextcloud CalDAV) alongside the existing Google Calendar sync. Shared logic is extracted into a common interface and a static helper, keeping both service implementations independent and self-contained.
+
+---
+
+## Architecture
+
+### Shared interface: `ICalendarService`
+
+Replaces `IGoogleCalendarService`. Both providers implement this interface:
+
+```csharp
+public interface ICalendarService
+{
+    Task<List<CalendarInfo>> GetCalendarsAsync();
+    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
+}
+
+public class CalendarInfo
+{
+    public string Id { get; set; }
+    public string Summary { get; set; }
+}
+```
+
+### Shared helper: `CalendarEventBuilder` (static class)
+
+Extracts pure data-transformation logic currently inside `GoogleCalendarService`:
+
+- `FormatTransportInfo(TransportConnection)` → string
+- `BuildEventContent(Shift, DateTimeOffset start, DateTimeOffset end, TransportConnection?)` → returns a shared `CalendarEventData` record (summary + description)
+
+No orchestration logic lives here — just pure functions with no side effects.
+
+### `GoogleCalendarService`
+
+- Implements `ICalendarService`
+- Refactored to use `CalendarEventBuilder` for event content
+- All Google client library calls remain here
+- `GetCalendarsAsync()` returns `List<CalendarInfo>` mapped from Google's `CalendarListEntry`
+- Identification of owned events via `ExtendedProperties.Private["shiftSchedulerEvent"]` unchanged
+
+### `NextcloudCalendarService`
+
+- Implements `ICalendarService`
+- Uses `HttpClient` with Basic Auth (username + app password from configuration)
+- CalDAV operations:
+  - **List calendars**: `PROPFIND` on `/remote.php/dav/calendars/<username>/`, depth 1, parse `displayname` and `resourcetype`
+  - **Delete existing events in range**: `REPORT` (CalDAV `calendar-query`) to find VEVENTs with `X-SHIFT-SCHEDULER:true` property in the date range, then `DELETE` each by URL
+  - **Create events**: `PUT` a VEVENT (iCalendar format using the existing `Ical.Net` dependency) to `<calendar-url>/<uid>.ics`. Each event includes `X-SHIFT-SCHEDULER:true` as a custom property for future identification.
+- UIDs are generated deterministically: `shift-<shiftName>-<dateISO>-<morning|afternoon>` to allow idempotent re-sync if needed
+- Retry logic mirrors Google's exponential backoff, extracted from `GoogleCalendarService` into a shared `RetryHelper` static method in `CalendarEventBuilder`
+
+---
+
+## Controller
+
+`ShiftController` is updated:
+
+- Both `IGoogleCalendarService` and `INextcloudCalendarService` injected. These are empty marker interfaces that extend `ICalendarService`, registered with their concrete types in DI. This avoids DI ambiguity while keeping the shared interface as the contract:
+  ```csharp
+  public interface IGoogleCalendarService : ICalendarService { }
+  public interface INextcloudCalendarService : ICalendarService { }
+  ```
+- Old endpoints `GET /api/shift/google_calendars` and `POST /api/shift/sync_to_google_calendar` are **replaced** by:
+  - `GET /api/shift/calendars?provider=google|nextcloud` → `List<CalendarInfo>`
+  - `POST /api/shift/sync_to_calendar` → body: `{ Provider, CalendarId, Year, Month }`
+- Provider routing via a `switch` on the `provider` string — no factory abstraction
+
+---
+
+## Client UI
+
+### Buttons
+
+Two separate sync buttons in `Index.razor`:
+- **Sync to Google Calendar** (existing, unchanged label)
+- **Sync to Nextcloud** (new)
+
+Both buttons are disabled while `_isSyncing` is true.
+
+### State
+
+New field `_syncProvider` (`"google"` or `"nextcloud"`) tracks which provider triggered the current flow.
+
+Existing `_availableCalendars`, `_showCalendarSelector`, and the calendar picker modal are reused for both providers. The modal title changes to "Select Google Calendar" or "Select Nextcloud Calendar" based on `_syncProvider`.
+
+### Client DTO
+
+`GoogleCalendar` record on the client is renamed to `CalendarInfo` to match the server DTO.
+
+---
+
+## Configuration
+
+### `appsettings.json` (placeholder — committed to git)
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "YOUR_NEXTCLOUD_USERNAME",
+  "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+}
+```
+
+### Production appsettings (not committed, mounted via Docker)
+
+Contains real values for `BaseUrl`, `Username`, and `AppPassword`.
+
+Read in `Program.cs` via `IConfiguration`, injected into `NextcloudCalendarService` — same pattern as `Authentication:Google`.
+
+### `DOCKER_CONFIG.md`
+
+Add a note that the `Nextcloud` section must be populated in the production `appsettings.json`.
+
+---
+
+## Dependencies
+
+No new NuGet packages required:
+- CalDAV HTTP calls: plain `HttpClient`
+- iCalendar VEVENT generation: `Ical.Net` (already in `ShiftScheduler.Services.csproj`)
+- XML parsing for CalDAV responses: `System.Xml` (built-in)
+
+---
+
+## Out of scope
+
+- Two-way sync (read events from calendar back into the scheduler)
+- Nextcloud OAuth — app password is sufficient for a personal instance
+- Caching of calendar lists