event_schedule (see event library), which essentially operates as a
deferred event_notify. event_schedule accepts a notification and a
firing time, alters the notification to change the type attribute to
EVENT_SCHEDULE and add a firing time attribute, and then sends the
altered notification using event_notify.  The event scheduler
subscribes to EVENT_SCHEDULE notifications. As they arrive,
it restores the type in the notification to that of the
original event and enqueues the notification in a priority queue
for firing at the indicated time. When the time arrives, the
scheduler removes the notification from the queue and resends it
using event_notify.

With these changes, the event system now supports dynamic events.