Could there be an option to use a fixed amount of days in advance in stead of using a date selected field for the closing date? The fix amount of days must be configurable, e.g. 5 days. Then an option in Create event where you either choose the date (as now) or a button fixed 5 days in advance (new).
It is quite time consuming to enter the closing date on each event when in reality this is a couple of days before the event.