Changes of the key properties (id, start, end, resource, text) doesn't have an immediate effect because it would cause problems if you modified events that are already visible in the calendar/scheduler.
The changes are stored internally and the DayPilot.Event object is marked as dirty.
The change is committed when you call dp.events.update(e). This is the recommended way.
You can also commit the changes manually using e.commit().
See also:
http://api.daypilot.org/daypilot-event-start/
http://api.daypilot.org/daypilot-scheduler-events-update/