An "active" calendar automatically analyzes a user's calendar entries and sends machine-readable messages to destinations appropriate to content of the calendar entry. A group of event categories is established, each category specifying one class of anticipated calendar entry. An action rule database pre-associates each event category with one or more message formats each having a content and a destination. The action rule database also contains data identifying sources containing the content and destination for each message format. These sources include records of the action rule database itself, subparts of calendar entries of the pre-associated event category, one or more other databases, or a combination of the foregoing. After the calendar receives a user-submitted computer calendar entry describing a planned event, it identifies one of the event categories of the established group that classifies the planned event. For each message format pre-associated with the identified event category, the calendar determines the content and destination for the message as specified by the action rule database, and transmits the message to the destination.