do I undersatnd correctly that the opening time was in the calendar, but the closing time was not?
Maybe the "hotpot_maxeventlength" configuration setting is relevant here (Settings -> Site administration -> Plugins -> Activity modules -> HotPot -> Maximum number of days for a single calendar event)
The default setting is 5 days. If the period between the open and close times are less than that, only one calendar event will be added. I think this is what happened in your case.
Probably your fix is to reset the "hotpot_maxeventlength" setting to "1". Any HotPot activities that you create or edit after you have changed the setting and whose close date is more than one day after the open date, should create two calendar events.
hope that helps!