Fix timezone handling across all calendar providers and tools (#41)

- Fix Google CreateEventAsync/UpdateEventAsync: replace DateTimeOffset(DateTime)
  constructor (which embeds local system UTC offset) with DateTimeRaw string
  (no offset), so Google Calendar honors the timeZone field rather than
  treating the time as UTC
- Fix UpdateEventAsync in all providers: replace TimeZoneInfo.Local.Id with
  the caller-supplied timeZone parameter; add timeZone to IProviderService
  interface and all implementations (ICS/JSON stubs included)
- Add missing UpdateEventTool and UpdateEventToolTests
- Fix GetCalendarEventsTool: replace DateTime.Today (server local) with
  TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tz).Date so "today"
  defaults to the caller's timezone, not the server's
- Fix Google GetCalendarEventsAsync: use DateTimeOffset(start, TimeSpan.Zero)
  instead of DateTimeOffset(start) to make UTC intent explicit and
  server-TZ-independent
- Fix M365/OutlookCom GetCalendarEventDetailsAsync: use ParseM365DateTime()
  helper (already used in the list path) instead of raw DateTime.TryParse,
  so the event timezone field is honored when mapping Start/End
- Fix JsonCalendarProviderService: use DateTimeStyles.RoundtripKind when
  parsing CreatedDateTime/LastModifiedDateTime so embedded UTC offsets are
  preserved before converting to universal time

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: rockfordlhotka

src/CalendarMcp.Core/Providers/GoogleProviderService.cs

@@ -550,8 +550,8 @@ public async Task<IEnumerable<CalendarEvent>> GetCalendarEventsAsync(
             var targetCalendarId = calendarId ?? "primary";
 
             var request = service.Events.List(targetCalendarId);
-            request.TimeMinDateTimeOffset = new DateTimeOffset(start);
-            request.TimeMaxDateTimeOffset = new DateTimeOffset(end);
+            request.TimeMinDateTimeOffset = new DateTimeOffset(start, TimeSpan.Zero);
+            request.TimeMaxDateTimeOffset = new DateTimeOffset(end, TimeSpan.Zero);
             request.MaxResults = count;
             request.OrderBy = EventsResource.ListRequest.OrderByEnum.StartTime;
             request.SingleEvents = true;
@@ -696,12 +696,12 @@ public async Task<string> CreateEventAsync(
                 Location = location,
                 Start = new EventDateTime
                 {
-                    DateTimeDateTimeOffset = new DateTimeOffset(start),
+                    DateTimeRaw = start.ToString("yyyy-MM-ddTHH:mm:ss"),
                     TimeZone = timeZone ?? "UTC"
                 },
                 End = new EventDateTime
                 {
-                    DateTimeDateTimeOffset = new DateTimeOffset(end),
+                    DateTimeRaw = end.ToString("yyyy-MM-ddTHH:mm:ss"),
                     TimeZone = timeZone ?? "UTC"
                 }
             };
@@ -728,14 +728,15 @@ public async Task<string> CreateEventAsync(
     }
 
     public async Task UpdateEventAsync(
-        string accountId, 
-        string calendarId, 
-        string eventId, 
-        string? subject = null, 
-        DateTime? start = null, 
-        DateTime? end = null, 
-        string? location = null, 
-        List<string>? attendees = null, 
+        string accountId,
+        string calendarId,
+        string eventId,
+        string? subject = null,
+        DateTime? start = null,
+        DateTime? end = null,
+        string? location = null,
+        List<string>? attendees = null,
+        string? timeZone = null,
         CancellationToken cancellationToken = default)
     {
         var credential = await GetCredentialAsync(accountId, cancellationToken);
@@ -764,16 +765,16 @@ public async Task UpdateEventAsync(
             {
                 existingEvent.Start = new EventDateTime
                 {
-                    DateTimeDateTimeOffset = new DateTimeOffset(start.Value),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    DateTimeRaw = start.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
             if (end.HasValue)
             {
                 existingEvent.End = new EventDateTime
                 {
-                    DateTimeDateTimeOffset = new DateTimeOffset(end.Value),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    DateTimeRaw = end.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
             if (attendees != null)

src/CalendarMcp.Core/Providers/IcsProviderService.cs

@@ -301,7 +301,7 @@ public Task UpdateEventAsync(
         string accountId, string calendarId, string eventId,
         string? subject = null, DateTime? start = null, DateTime? end = null,
         string? location = null, List<string>? attendees = null,
-        CancellationToken cancellationToken = default)
+        string? timeZone = null, CancellationToken cancellationToken = default)
         => throw new NotSupportedException("ICS provider is read-only.");
 
     public Task DeleteEventAsync(

src/CalendarMcp.Core/Providers/JsonCalendarProviderService.cs

@@ -357,7 +357,7 @@ public Task UpdateEventAsync(
         string accountId, string calendarId, string eventId,
         string? subject = null, DateTime? start = null, DateTime? end = null,
         string? location = null, List<string>? attendees = null,
-        CancellationToken cancellationToken = default)
+        string? timeZone = null, CancellationToken cancellationToken = default)
         => throw new NotSupportedException("JSON calendar provider is read-only.");
 
     public Task DeleteEventAsync(
@@ -488,16 +488,16 @@ private CalendarEvent MapToCalendarEvent(
         // Parse categories
         var categories = entry.Categories ?? new List<string>();
 
-        // Parse dates
+        // Parse dates — use RoundtripKind to honour any UTC/offset info embedded in the string
         DateTime? createdDateTime = null;
         if (!string.IsNullOrEmpty(entry.CreatedDateTime) &&
-            DateTime.TryParse(entry.CreatedDateTime, out var created))
-            createdDateTime = created.ToUniversalTime();
+            DateTime.TryParse(entry.CreatedDateTime, null, System.Globalization.DateTimeStyles.RoundtripKind, out var created))
+            createdDateTime = created.Kind == DateTimeKind.Unspecified ? created : created.ToUniversalTime();
 
         DateTime? lastModifiedDateTime = null;
         if (!string.IsNullOrEmpty(entry.LastModifiedDateTime) &&
-            DateTime.TryParse(entry.LastModifiedDateTime, out var modified))
-            lastModifiedDateTime = modified.ToUniversalTime();
+            DateTime.TryParse(entry.LastModifiedDateTime, null, System.Globalization.DateTimeStyles.RoundtripKind, out var modified))
+            lastModifiedDateTime = modified.Kind == DateTimeKind.Unspecified ? modified : modified.ToUniversalTime();
 
         // Detect online meeting URL from body or webLink
         string? onlineMeetingUrl = null;

src/CalendarMcp.Core/Providers/M365ProviderService.cs

@@ -637,8 +637,8 @@ public async Task<IEnumerable<CalendarEvent>> GetCalendarEventsAsync(
                 AccountId = accountId,
                 CalendarId = calendarId ?? "primary",
                 Subject = evt.Subject ?? string.Empty,
-                Start = DateTime.TryParse(evt.Start?.DateTime, out var startDt) ? startDt : DateTime.MinValue,
-                End = DateTime.TryParse(evt.End?.DateTime, out var endDt) ? endDt : DateTime.MinValue,
+                Start = ParseM365DateTime(evt.Start),
+                End = ParseM365DateTime(evt.End),
                 Location = evt.Location?.DisplayName ?? string.Empty,
                 Body = evt.Body?.Content ?? string.Empty,
                 BodyFormat = evt.Body?.ContentType == BodyType.Html ? "html" : "text",
@@ -912,14 +912,15 @@ public async Task<string> CreateEventAsync(
     }
 
     public async Task UpdateEventAsync(
-        string accountId, 
-        string calendarId, 
-        string eventId, 
-        string? subject = null, 
-        DateTime? start = null, 
-        DateTime? end = null, 
-        string? location = null, 
-        List<string>? attendees = null, 
+        string accountId,
+        string calendarId,
+        string eventId,
+        string? subject = null,
+        DateTime? start = null,
+        DateTime? end = null,
+        string? location = null,
+        List<string>? attendees = null,
+        string? timeZone = null,
         CancellationToken cancellationToken = default)
     {
         var token = await GetAccessTokenAsync(accountId, cancellationToken);
@@ -945,7 +946,7 @@ public async Task UpdateEventAsync(
                 eventUpdate.Start = new DateTimeTimeZone
                 {
                     DateTime = start.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
 
@@ -954,7 +955,7 @@ public async Task UpdateEventAsync(
                 eventUpdate.End = new DateTimeTimeZone
                 {
                     DateTime = end.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
 

src/CalendarMcp.Core/Providers/OutlookComProviderService.cs

@@ -628,8 +628,8 @@ public async Task<IEnumerable<CalendarEvent>> GetCalendarEventsAsync(
                 AccountId = accountId,
                 CalendarId = calendarId ?? "primary",
                 Subject = evt.Subject ?? string.Empty,
-                Start = DateTime.TryParse(evt.Start?.DateTime, out var startDt) ? startDt : DateTime.MinValue,
-                End = DateTime.TryParse(evt.End?.DateTime, out var endDt) ? endDt : DateTime.MinValue,
+                Start = ParseM365DateTime(evt.Start),
+                End = ParseM365DateTime(evt.End),
                 Location = evt.Location?.DisplayName ?? string.Empty,
                 Body = evt.Body?.Content ?? string.Empty,
                 BodyFormat = evt.Body?.ContentType == BodyType.Html ? "html" : "text",
@@ -902,14 +902,15 @@ public async Task<string> CreateEventAsync(
     }
 
     public async Task UpdateEventAsync(
-        string accountId, 
-        string calendarId, 
-        string eventId, 
-        string? subject = null, 
-        DateTime? start = null, 
-        DateTime? end = null, 
-        string? location = null, 
-        List<string>? attendees = null, 
+        string accountId,
+        string calendarId,
+        string eventId,
+        string? subject = null,
+        DateTime? start = null,
+        DateTime? end = null,
+        string? location = null,
+        List<string>? attendees = null,
+        string? timeZone = null,
         CancellationToken cancellationToken = default)
     {
         var token = await GetAccessTokenAsync(accountId, cancellationToken);
@@ -935,7 +936,7 @@ public async Task UpdateEventAsync(
                 eventUpdate.Start = new DateTimeTimeZone
                 {
                     DateTime = start.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
 
@@ -944,7 +945,7 @@ public async Task UpdateEventAsync(
                 eventUpdate.End = new DateTimeTimeZone
                 {
                     DateTime = end.Value.ToString("yyyy-MM-ddTHH:mm:ss"),
-                    TimeZone = TimeZoneInfo.Local.Id
+                    TimeZone = timeZone ?? "UTC"
                 };
             }
 

src/CalendarMcp.Core/Services/IProviderService.cs

@@ -93,6 +93,7 @@ Task UpdateEventAsync(
         DateTime? end = null,
         string? location = null,
         List<string>? attendees = null,
+        string? timeZone = null,
         CancellationToken cancellationToken = default);
 
     Task DeleteEventAsync(

src/CalendarMcp.Core/Tools/GetCalendarEventsTool.cs

@@ -35,7 +35,7 @@ public async Task<string> GetCalendarEvents(
             });
         }
 
-        var resolvedStart = startDate ?? DateTime.Today;
+        var resolvedStart = startDate ?? TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, tz).Date;
         var resolvedEnd = endDate ?? resolvedStart.AddDays(7);
 
         logger.LogInformation("Getting calendar events: startDate={StartDate}, endDate={EndDate}, accountId={AccountId}, count={Count}, timeZone={TimeZone}",

src/CalendarMcp.Core/Tools/UpdateEventTool.cs

@@ -0,0 +1,66 @@
+using System.ComponentModel;
+using System.Text.Json;
+using CalendarMcp.Core.Services;
+using Microsoft.Extensions.Logging;
+using ModelContextProtocol.Server;
+
+namespace CalendarMcp.Core.Tools;
+
+/// <summary>
+/// MCP tool for updating calendar events
+/// </summary>
+[McpServerToolType]
+public sealed class UpdateEventTool(
+    IAccountRegistry accountRegistry,
+    IProviderServiceFactory providerFactory,
+    ILogger<UpdateEventTool> logger)
+{
+    [McpServerTool, Description("Update an existing calendar event. Always pass the timeZone parameter using the user's local IANA timezone (e.g. `America/Chicago`, `America/New_York`, `Europe/London`) when updating start/end times so events are updated at the correct local time.")]
+    public async Task<string> UpdateEvent(
+        [Description("Account ID that owns the event. Obtain from list_accounts.")] string accountId,
+        [Description("Calendar ID that contains the event. Obtain from list_calendars.")] string calendarId,
+        [Description("Event ID to update. Obtain from get_calendar_events or get_calendar_event_details.")] string eventId,
+        [Description("New event subject/title")] string? subject = null,
+        [Description("New event start date and time (ISO 8601 format)")] DateTime? start = null,
+        [Description("New event end date and time (ISO 8601 format)")] DateTime? end = null,
+        [Description("New event location")] string? location = null,
+        [Description("New list of attendee email addresses")] List<string>? attendees = null,
+        [Description("IANA timezone name for the event times (e.g. `America/Chicago`, `America/New_York`, `Europe/London`). Required when updating start or end times.")] string? timeZone = null)
+    {
+        logger.LogInformation("Updating event: eventId={EventId}, accountId={AccountId}, calendarId={CalendarId}",
+            eventId, accountId, calendarId);
+
+        try
+        {
+            var account = await accountRegistry.GetAccountAsync(accountId);
+            if (account == null)
+            {
+                return JsonSerializer.Serialize(new
+                {
+                    error = $"Account '{accountId}' not found"
+                });
+            }
+
+            var provider = providerFactory.GetProvider(account.Provider);
+            await provider.UpdateEventAsync(
+                accountId, calendarId, eventId, subject, start, end, location, attendees, timeZone, CancellationToken.None);
+
+            return JsonSerializer.Serialize(new
+            {
+                success = true,
+                eventId,
+                accountUsed = accountId,
+                calendarUsed = calendarId
+            }, new JsonSerializerOptions { WriteIndented = true });
+        }
+        catch (Exception ex)
+        {
+            logger.LogError(ex, "Error in update_event tool");
+            return JsonSerializer.Serialize(new
+            {
+                error = "Failed to update event",
+                message = ex.Message
+            });
+        }
+    }
+}

src/CalendarMcp.Tests/Tools/UpdateEventToolTests.cs

@@ -0,0 +1,67 @@
+using System.Text.Json;
+using CalendarMcp.Core.Models;
+using CalendarMcp.Core.Services;
+using CalendarMcp.Core.Tools;
+using CalendarMcp.Tests.Helpers;
+using Microsoft.Extensions.Logging.Abstractions;
+using Rocks;
+
+namespace CalendarMcp.Tests.Tools;
+
+[TestClass]
+public class UpdateEventToolTests
+{
+    private static readonly DateTime Start = new(2025, 6, 1, 10, 0, 0);
+    private static readonly DateTime End = new(2025, 6, 1, 11, 0, 0);
+
+    [TestMethod]
+    public async Task UpdateEvent_Success()
+    {
+        var account = TestData.CreateAccount(id: "acc-1", provider: "microsoft365");
+
+        var regExp = new IAccountRegistryCreateExpectations();
+        regExp.Setups.GetAccountAsync("acc-1")
+            .ReturnValue(Task.FromResult<AccountInfo?>(account));
+
+        var provExp = new IProviderServiceCreateExpectations();
+        provExp.Setups.UpdateEventAsync(
+            "acc-1", "cal-1", "ev-1",
+            Arg.Any<string?>(), Arg.Any<DateTime?>(), Arg.Any<DateTime?>(),
+            Arg.Any<string?>(), Arg.Any<List<string>?>(), Arg.Any<string?>(), Arg.Any<CancellationToken>())
+            .ReturnValue(Task.CompletedTask);
+
+        var factExp = new IProviderServiceFactoryCreateExpectations();
+        factExp.Setups.GetProvider("microsoft365").ReturnValue(provExp.Instance());
+
+        var tool = new UpdateEventTool(regExp.Instance(), factExp.Instance(),
+            NullLogger<UpdateEventTool>.Instance);
+
+        var result = await tool.UpdateEvent("acc-1", "cal-1", "ev-1", subject: "Updated", start: Start, end: End, timeZone: "America/Chicago");
+        var doc = JsonDocument.Parse(result);
+
+        Assert.IsTrue(doc.RootElement.GetProperty("success").GetBoolean());
+        Assert.AreEqual("ev-1", doc.RootElement.GetProperty("eventId").GetString());
+
+        regExp.Verify();
+        factExp.Verify();
+        provExp.Verify();
+    }
+
+    [TestMethod]
+    public async Task UpdateEvent_AccountNotFound_ReturnsError()
+    {
+        var regExp = new IAccountRegistryCreateExpectations();
+        regExp.Setups.GetAccountAsync("nonexistent")
+            .ReturnValue(Task.FromResult<AccountInfo?>(null));
+
+        var factExp = new IProviderServiceFactoryCreateExpectations();
+        var tool = new UpdateEventTool(regExp.Instance(), factExp.Instance(),
+            NullLogger<UpdateEventTool>.Instance);
+
+        var result = await tool.UpdateEvent("nonexistent", "cal-1", "ev-1");
+        var doc = JsonDocument.Parse(result);
+
+        Assert.AreEqual("Account 'nonexistent' not found", doc.RootElement.GetProperty("error").GetString());
+        regExp.Verify();
+    }
+}