I'm building software that has to integrate with Outlook and Google calendars, what API should I use?

If you need to integrate with both Outlook and Google Calendar, the usual choice is:

Use the provider APIs directly

• Google Calendar API for Google accounts
• Microsoft Graph API for Outlook / Microsoft 365 / Exchange Online calendars

That’s generally the best approach because each provider has its own auth, permissions, event model, and sync behavior.

Recommended setup

Build a small abstraction layer in your app, for example:

• listCalendars()
• listEvents()
• createEvent()
• updateEvent()
• deleteEvent()
• watchChanges() / webhook handling

Then implement:

• one adapter for Google Calendar API
• one adapter for Microsoft Graph Calendar API

Why not use a single universal API?

There are calendar aggregation APIs like:

• Nylas
• Cronofy
• Fantastical-like sync platforms (less for app integration)
• others in the calendar infrastructure space

These can save time if you want:

• one unified API
• fewer OAuth integrations
• normalized event objects
• built-in sync/webhook handling

But they add:

• vendor dependency
• extra cost
• possible limitations vs direct provider APIs

My recommendation

• If you’re building something serious and want maximum control:
  Use Google Calendar API + Microsoft Graph API directly.
• If you want fastest time-to-market with less integration work:
  Use a calendar aggregation service like Nylas or Cronofy.

Practical note

For Outlook, don’t use the older Outlook REST API — it’s deprecated.
Use Microsoft Graph instead.

If you want, I can also give you:

1. a feature-by-feature comparison of Google vs Microsoft Graph, or
2. a suggested architecture for supporting both calendars cleanly.