1. A computer-implemented method comprising: storing a calendar event in a calendar system, wherein the calendar event comprises: dynamic rich media content; and data that specifies an active time interval for display of the dynamic rich media content; presenting a rich-media-enabled calendar display interface; determining whether a current time is within the active time interval for the calendar event; in response to determining that the current time is within the active time interval for the calendar event, automatically causing the calendar event to be displayed with the dynamic rich media content for the calendar event via the rich-media-enabled calendar display interface; in response to determining that the current time is not within the active time interval for the calendar event, instead of causing the calendar event to be displayed with the dynamic rich media content for the calendar event via the rich-media-enabled calendar display interface, causing the calendar event to be displayed without the dynamic rich media content for the calendar event via the rich-media-enabled calendar display interface; wherein the method is performed by one or more computing devices.