This function is quite helpful if user track time after midnight frequently or for user have work shift.
It affects duration calculation on "Today" tab and "Report" tab. ATracker will always show data in normal 24 hours setting on "Calendar" and "History" tab.
For example, if user normally sleeps at 2AM every day, he can set the day start time to 3 or 4AM. By doing so, the duration before 3 / 4AM will be calculated as the day before.