1. A computer system comprising: at least one processor; and memory comprising instructions stored thereon that when executed by at least one processor cause at least one processor to perform acts comprising: automatically calculating a synchronization window for synchronizing data for a calendar in a pre-existing 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, the calculation of the synchronization window using a current time; sending to the calendar data server a request for synchronization data for calendar items for the calendar with calendar times that are at least partially within the synchronization window; receiving from the calendar data server one or more responses to the request, the one or more responses comprising received records for calendar items that are at least partially within the synchronization window, the received records comprising a master record of a recurring calendar item and an instance record of an occurrence of the recurring calendar item; incorporating the received records for the calendar items in the client calendar data store; and in response to receiving user input while the client computer system is not connected to the calendar data server, displaying a calendar view with one or more representations of the calendar items on an output device of the client computer system, the displaying of the calendar view being performed while the client computer system is not connected to the calendar data server.