A synchronization window for synchronizing data for a calendar in a client calendar data store on a calendar data client computer system with data for the calendar in a server calendar data store on a calendar data server computer system can be calculated using a current time. A request for synchronization data for calendar items for the calendar with calendar times that are within the synchronization window can be sent to the calendar data server. One or more responses to the request can be received from the calendar data server. The response(s) can include received records for calendar items that are at least partially within the synchronization window. The received records can include a master record of a recurring calendar item and an instance record of an occurrence of the recurring calendar item. The received records for the calendar items can be incorporated in the client calendar data store.