How can I extend Calendar on Android?

Question

I mean, is there any way to

• add more functionality
• add/remove events
• synchronize with other then Google calendars/formats (authorized)

Answer 1

There is a content provider available for Calendar entries: android.provider.Calendar.CONTENT_URI (see brief documentation here).

Though, I'd consider using the Google Calendar Data API. You should be able to authenticate the user using AccountManager#getAuthToken and edit calendar entries in very rich and well-documented ways.