CalendarSync

CalendarSync is a lightweight .NET-based background application that performs a one-way sync from Microsoft Outlook to Apple iCloud Calendar using CalDAV. As a COM-Interop application it is optimized to run silently via a Scheduled Task on Windows systems - without relying on Microsoft Graph or full Exchange integration.

Features

• One-Way syncs events from local Outlook calendar to Apple iCloud calendar
• Adds a 10-minute and 3-minute alarm notification for timed events (no reminders on all-day or multi-day entries)
• Runs silently and logs to sync.log
• Designed for restricted corporate environments — no UI required
• Tray icon with status tooltip
• Ability to sync multiple source calendars into the target:
  1. Ideal for multiple machines handed out by the consultancy and client
  2. Cleanly separates and manages multiple sources
  3. Ability to visually 'tag' entries from sources

Requirements

• Windows with Outlook installed and configured
• iCloud Calendar with CalDAV access
• .NET 8.0 or newer
• Basic access to Scheduled Tasks (admin rights only for registration)

Quickstart

1. Build & Deploy

• Compile the app (Release mode).
• Copy the output (.exe + config.json) to a permanent path, e.g.:

C:\CalendarSync\

• Fill in config.json with your iCloud credentials and calendar info:

{
    "ICloudCalDavUrl": "https://caldav.icloud.com",
    "ICloudUser": "your_apple_id@icloud.com",
    "ICloudPassword": "app-specific-password",
    "PrincipalId": "XXXXXXXXX",
    "WorkCalendarId": "YYYYYYYYY",
    "InitialWaitSeconds": 60,
    "SyncIntervalMinutes": 3,
    "SyncDaysIntoFuture": 30,
    "SyncDaysIntoPast": 30,
    "LogLevel": "Information",
    "SourceId": "in_case_you_want_to_sync_from_multiple_calendars",
    "EventTag": "this_marks_an_entry_with_a_prefix_e.g_ [COMPANY]",
    "SourceTimeZoneId": "Europe/Berlin",
    "TargetTimeZoneId": "Europe/Berlin"
}

Use a browser Dev Tools or CalDAV client to retrieve PrincipalId and WorkCalendarId.

SourceTimeZoneId and TargetTimeZoneId are optional. When omitted, the service falls back to the host operating system's local timezone for both the Outlook source and the iCloud destination.

2. Register as a Scheduled Task

Manual Method:

1. Open Task Scheduler → Create Task
2. General Tab:
   • Name: CalendarSync
   • Run only when user is logged on
   • Run with highest privileges
3. Triggers Tab: Add → At log on
4. Actions Tab:
   • Start a program → C:\CalendarSync\CalendarSync.exe

Logs

Logs are written to:

C:\CalendarSync\sync.log

High level events are also written to the Windows Event Log under the "Application" log. Run the program once as administrator to register the "CalendarSync" event source.

Security

• Does not store or sync from iCloud to Outlook
• Passwords handled via basic auth over HTTPS

Outlook COM Reliability

• Uses a dedicated STA thread with timeouts to prevent Outlook UI hangs.

Notes

• Outlook must be configured and ready on the host
• iCloud must be accessible via CalDAV (app-specific password required)

License

MIT — use at your own risk and discretion.