Create the Date “WorkshopStartEndTimeFreeze”. After that date the (workshop, tutorial, demo, etc) schedule editor will refuse to save an event that starts before the parent workshop's event start time or after the parent's end time. To modify the start and end times allowed, you must modify the parent event's start and end time.
If the event's parent (Workshopapplications.event) has no start or end time, then there is no enforcement, even after the deadline.