CalSync

A lightweight macOS menu bar app that syncs calendar events between two calendars connected to your Mac (e.g., Gmail ↔ Microsoft). It uses Apple's EventKit framework, so any calendar visible in the macOS Calendar app can be used — no API keys or third-party services required.

Requirements

• macOS 14.0+
• Swift 5.9+
• Both calendars must be added to macOS via System Settings → Internet Accounts

Build

# Debug build
swift build

# Release build + .app bundle
./bundle.sh

The bundle script compiles a release build and packages it into build/CalSync.app with the proper Info.plist (which hides the app from the Dock).

Install

cp -r build/CalSync.app /Applications/

Usage

First Launch

1. Run the app:

   open build/CalSync.app

2. A calendar icon (🕐) appears in the menu bar — the app has no Dock icon.
3. macOS will prompt you to grant Calendar access. Allow it.

Configure

1. Click the menu bar icon → Settings…
2. Source Calendar — the calendar to read events from (e.g., your Gmail calendar).
3. Destination Calendar — the calendar to copy events to (e.g., your Microsoft/Outlook calendar).
4. Sync Direction:
   • One-way (A → B) — events flow from source to destination only.
   • Two-way (A ↔ B) — events sync in both directions.
5. Auto-Sync Interval — how often the app syncs automatically, in minutes (default: 30, set to 0 to disable).
6. Click Save.

Sync

• Manual: Click the menu bar icon → Sync Now.
• Automatic: Runs on the configured interval in the background.

The menu bar icon changes while a sync is in progress. The "Last synced" timestamp updates after each successful sync.

Launch at Login

Click the menu bar icon → Launch at Login to toggle starting CalSync automatically when you log in.

Quit

Click the menu bar icon → Quit CalSync.

What Gets Synced

Field	Synced
Title	✅
Start / End time	✅
All-day flag	✅
Location	✅
Structured location	✅
Notes	✅
URL	✅
Availability	✅
Alarms / Reminders	✅
Recurrence rules	✅
Attendees	⚠️ Appended as text in notes (read-only in EventKit)

How It Works

• Events are fetched from the source calendar for the next ~4 years (EventKit's maximum predicate range).
• Each synced event is tracked by its calendarItemExternalIdentifier in a local state file at ~/Library/Application Support/CalSync/sync-state.json.
• New events in the source are created in the destination.
• Modified events (source changed since last sync) are updated in the destination.
• Deleted events (no longer in the source) are removed from the destination.
• Deduplication: before creating, the app checks for existing events with the same title and start time (±1 minute) to avoid duplicates.
• Two-way sync runs the same logic in both directions, skipping events that were created by sync to prevent loops.

Files

Path	Purpose
Sources/CalSync/main.swift	App entry point
Sources/CalSync/AppDelegate.swift	Menu bar UI, timer, window management
Sources/CalSync/CalendarSyncService.swift	EventKit sync engine
Sources/CalSync/Settings.swift	UserDefaults-backed preferences
Sources/CalSync/SyncState.swift	JSON persistence for event ID mappings
Sources/CalSync/SettingsWindowController.swift	Settings window UI
Sources/CalSync/Info.plist	App metadata (LSUIElement, permissions)
bundle.sh	Packages the binary into a .app bundle