What is calendar sync?
Calendar sync connects your app's bookings, appointments, or events to users' personal or work calendars — Google Calendar, Outlook, or Apple Calendar. When a booking is made in your app, an event automatically appears in the user's calendar. When the user accepts, reschedules, or cancels in their calendar, your app knows about it.
One-way sync (your app pushes events to the calendar) is the simpler case: your app creates, updates, and deletes events via the calendar provider's API. Two-way sync (changes in the calendar reflect back in your app) is significantly more complex — it requires polling or webhooks to detect calendar-side changes and reconciling those changes with your app's booking state.
The two main provider APIs are Google Calendar API (OAuth 2.0, part of Google Workspace) and Microsoft Graph API (for Outlook, Microsoft 365, and Teams calendar). Supporting both doubles the integration surface but covers most of the Australian business market.
When does your app need it?
- Users make bookings in your app and expect those appointments to appear in their work calendar automatically
- Your app is a scheduling or appointment tool and calendar integration is a baseline expectation
- Field staff need their job schedule visible in the same calendar they use for everything else
- You want to reduce no-shows by ensuring appointments are properly calendared with reminders
- Your users are in corporate environments where Microsoft 365 / Outlook is the standard calendar
- You need to detect when a user has a conflicting calendar event before confirming a booking
How much does it cost?
Adding calendar sync typically adds 5–11 hours of development — roughly $1,000–$2,000 AUD.
One-way sync (app → calendar) sits at the lower end. Two-way sync, supporting both Google and Microsoft, and conflict detection push toward the higher end. If you need to support CalDAV as an open standard (for Apple Calendar and other clients), that adds further complexity.
How it's typically built
For Google Calendar, users authorise your app via OAuth 2.0. Your app receives a token it uses to call the Google Calendar API — creating events with title, time, location, description, and attendees. Webhook notifications (Google "push notifications") can inform your app of calendar-side changes for two-way sync.
For Microsoft 365 / Outlook, the Microsoft Graph API follows the same pattern via Azure AD OAuth. Graph API covers calendar events, Teams meetings, and rooms.
CalDAV is an open standard supported by Apple Calendar, Fastmail, and many others. It requires a different client library but can reach users who don't use Google or Microsoft.
Storing the external event ID returned by the calendar API is essential — it's how your app updates or deletes the correct calendar event later.
Questions to ask your developer
- One-way or two-way sync? Two-way sync is substantially more work — be clear on whether you need changes made in the calendar to reflect back in your app.
- Which providers are in scope? Google Calendar and Microsoft 365 each require separate integration; confirm which your users actually need.
- How are revoked permissions handled? Users can revoke OAuth access at any time; there should be a graceful fallback and re-authorisation prompt.
- What calendar data is stored on your servers? Be clear on what calendar content is retained — users are sensitive about calendar privacy.
- Is conflict checking in scope? Detecting existing calendar conflicts before confirming a booking requires read access to the user's calendar, not just write access.
See also: Booking system · Resource and staff scheduling · Availability management · App cost calculator