Stop your Macs from double-booking each other.
CALSync is a free, private menu bar app that syncs busy time between your own Macs — work and personal — so every calendar knows when you're actually free. No account, no Google, no third-party cloud: only anonymous hashed busy blocks ever leave your machine, synced through a tiny server you host yourself.
- Install CALSync on each Mac — your work Mac with its Exchange calendar, your personal Mac with iCloud, and so on.
- Pair them with one setup code. The first Mac generates a private sync code automatically; copy the setup code from its Settings and paste it into the wizard on the others. No sign-up, no OAuth.
- Busy time flows both ways. Each Mac writes title-only "busy" mirror events into a calendar of your choice, so anyone checking your free/busy on either side sees you're taken.
- Sync calendars between Macs from the menu bar — runs in the background
- Privacy-hashed sync: only start/end times, a short label, and hashed IDs leave the device — never titles, attendees, locations, or notes
- No account, no OAuth — pairing is a single setup code
- Self-hosted: a single-binary Bun + SQLite relay (runs anywhere, e.g. Railway)
- Exchange- and CalDAV-safe mirror markers that survive server round-trips
- Built-in Doctor diagnostics for connectivity, auth, and calendar permissions
- Signed and notarized (Developer ID); auto-updates via Sparkle
- Documented, platform-neutral sync protocol
What leaves your Mac: hashed identifiers, start/end timestamps, and your chosen short display label. What never leaves your Mac: event titles, attendees, locations, notes, calendar names, raw event IDs. The relay can't reconstruct your schedule — it only passes opaque busy blocks between your own machines.
- Download the latest
CALSync-x.y.z.dmgfrom Releases. - Drag CALSync to Applications.
- On first launch, grant Calendar full access — CALSync needs it to read busy times and write mirror events.
- Follow the wizard: the first Mac just continues (it mints your sync code); additional Macs paste the setup code from the first Mac's Settings.
- macOS 14 (Sonoma) or newer, Apple silicon or Intel (universal binary)
- A small always-on server for the relay — a free-tier Railway service or any box that can run Bun (single binary, SQLite storage)
Does it work with Google, Exchange, iCloud, or CalDAV calendars? Yes — CALSync reads whatever is in Calendar.app via EventKit, and its mirror markers survive Exchange and CalDAV round-trips.
Can other people see my event details?
No. Titles, attendees, and locations never leave your Mac. Mirror events on
the other Mac are title-only labels like WX.
Is this for teams? No — CALSync syncs one person's multiple Macs. It's not a team scheduling tool and doesn't sync provider-to-provider.
Do I need an account? No accounts anywhere. Your sync code is the only credential, and you host the relay yourself.
Why is a server needed at all? Your Macs are rarely awake at the same time. The relay stores only hashed busy blocks so each Mac can catch up whenever it comes online.
- Protocol documentation: protocol.md
- Update feed (Sparkle appcast): https://itsklimov.github.io/CALSync/appcast.xml