In my experience it is just $30 per night and it can't be split and it doesn't carry over.

If you check in at 3PM for a 1 night stay you can use it that night OR in the morning.
If you check in at 1AM for a 1 night stay you can use it in the morning.
You can't use $15 at night and $15 in the morning (only $15 will credit).