Calendaring techniques and interfaces

The calendaring techniques and interfaces described herein provide access to calendar data stored in a server hosted calendar store to applications. The calendar data includes calendar events and tasks. In one aspect, an application program interface (API) retrieves an occurrence from a series of reoccurring calendar data upon request from an application. In another aspect, the API sends calendar data provided by the application to a server program that manages a calendar store for storage and queries the server program to retrieve calendar data requested by the application from the calendar store. In yet another aspect, the API sends notifications that the calendar store has changed to interested applications.