Show the start time of an event if there is no duration or no end time

Often there are events in schoollife, that have a known start time but the end time is unkown.

All in all there are three cases:
1: only the day is known, start time and end time is unknown (Display: Tuesday, 7 April)
2: event with a known day, start time and end time (Display: Tuesday, 7 April, 10:00>>12:00)
3: only the day and the start time is known and endtime is unknown (Display: Tuesday, 7 April, 10:00)

Case 3 is not supported by moodle.
So we need a combo-box to choose one of these cases in the event form to display the event better.