Setting time may make the event date span backward
Created attachment 336415 Calendar new event creation (video)
Setting a time for an 'All day' event may make the event dated backward. This seems to happen only for single day events.
See the attached screencast (The event is added on 29th Sept, when time is set, the end date sets to 28th)
Attachment 336415, "Calendar new event creation (video)":