Don't try to be too smart. That's only annoying to the user and does not help usability at all. Allow the user to enter an end time that is before the start time. He might want to adjust the start time later on or the current state is just a transition state and the user will change e.g. the year of the end date later on. We just step on his toes if we reset the value of the widget to the start date in these cases...