I'm pretty sure that this must be possible in some of their systems. If a flight scheduled to depart at 23:55 leaves 10 minutes late, it keeps the same flight number; but if it goes mechanical and leaves 10 hours late, it gets renumbered because they "can't have two different flights using the same number on the same day". So I must conclude that somewhere between 00:05 and 09:55 lies the start of a "new day".