Scheduling primitives
Calendar Sync
Calendar sync is the two-way connection between your scheduling tool and your calendar provider (Google Calendar, Outlook / Office 365, or Apple iCloud). It serves two functions: reading your existing calendar to know when you’re available, and writing new bookings back to your calendar so they appear alongside your other events.
How Two-Way Sync Works
Read (availability check): When a booker opens your booking page, the scheduler checks your connected calendar for existing events and marks those times as unavailable. A dentist appointment on Thursday at 2 PM won’t appear as a bookable slot — even though the scheduling tool didn’t create that event.
Write (booking confirmation): When someone books a slot, the scheduler creates a calendar event in your connected calendar. The event includes the booker’s name, their email, the meeting link (Zoom, Google Meet, etc.), and any custom details from the booking form.
Update and delete: If the booker reschedules or cancels, the scheduler updates or removes the calendar event. If you delete the event from your calendar directly, some tools (not all) detect this and mark the slot as cancelled.
Why It Matters for Double-Booking Prevention
The most common scheduling failure mode is a fragmented calendar: a personal Gmail calendar plus a work Outlook calendar plus a client-specific calendar. Most scheduling tools connect to one calendar by default, which means busy blocks on the other calendars are invisible to the scheduler.
Most professional tools allow connecting multiple calendars for availability checking even if they write bookings to only one. Calendly, for example, can check up to 6 calendars for conflicts while writing new bookings to one designated calendar. This prevents the classic double-book where a prospect grabs a slot that’s already blocked on the calendar the tool wasn’t watching.
Calendar Compatibility by Tool
| Tool | Google Calendar | Outlook / 365 | Apple iCloud | Notes |
|---|---|---|---|---|
| Calendly | Full 2-way | Full 2-way | Supported | Check up to 6 calendars for conflicts |
| Acuity | Full 2-way | Full 2-way | Better reliability than Calendly | Multi-staff: each staff member connects independently |
| Cal.com | Full 2-way | Full 2-way | Via CalDAV | Self-hosted: connect any CalDAV-compatible calendar |
| HoneyBook | Google only | No | No | Limited calendar integration |
iCal / CalDAV vs. OAuth
Two methods exist for calendar connection:
OAuth (recommended): A secure token-based connection. You click “Connect Google Calendar,” authorise the app, and the connection is maintained without sharing your password. Revocable from your Google account settings.
iCal feed: A one-way URL that exposes your calendar as a read-only feed. Less secure, less reliable for real-time two-way sync. Used by tools that don’t support OAuth for a specific calendar provider.
CalDAV: An open standard for calendar access. Used by Cal.com for Apple Calendar and other non-Google/Outlook providers. More flexible than iCal feed, requires slightly more setup.
The Multi-Calendar Problem
A common setup for a freelancer who recently went independent:
- Personal events: Google Calendar (personal Gmail)
- Client work: Outlook (client’s Microsoft 365 tenant)
- Family: Shared Apple Calendar with spouse
The scheduling tool can only see one calendar by default. A booker could theoretically grab a slot that’s blocked by a dentist appointment on the Apple Calendar that the scheduler can’t read.
The fix: use a tool that supports multi-calendar conflict checking (Calendly Standard and above, Acuity, Cal.com Teams). Connect all three calendars for conflict checking; designate one as the write target for new bookings.
Related Terms
- Buffer time — gaps before/after meetings that the synced calendar enforces
- Availability rules — hours and days the scheduler shows as bookable
- Booking page — where bookers see calendar-sync-derived availability
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