Yes they do expire midnight the day you check out, but we had problem with this past March with this. As we had to check out but checked into a new room in the same resort, since it was just for the night, we didnt put a credit card on file. So when we check out all of our credits didnt work. Supposedly the manger told us it was cause we didnt have a credit card on file and with the new system in place it deactivate our card. So he ended up transferring all of our credit to our new reservations, which end up working well, since we didnt have to use all the credits that day, although it over an hour to sort out.