Welcome to the frustration we all feel. Basically, just work out the unix datestamp for when you want the event to end, and then manually update the event in the database - cumbersome, but it works!
What gets me, is that start and end datestamps are used in the database.... why then cant there be a simple date and time picker on the front end for both start and end? That way we can say an event starts on monday 28th May at 22:47 and ends on the 29th May at 06:28!! Ok, extreme example but that level of flexibility is already supported in the database tables of this mod - the front end is just letting it down.