End time in event editor should be based on start time, not current time

Currently the preset end time (and the drop-down list with 30-min-aligned timestamps and "0" at the top) is calculated based on the current time, which is useless if the event is not beginning now or at least today.

Possible solutions:
1. A script should change the end time when changing the start time, if the end time hasn’t been edited.
2. The editor should have an invalid value like “??:??” as the default.