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]"
}

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

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

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.