I've made my National reservations first when rates were low and then my plane tickets months later and was off on the time by a few hours and it wasn't a problem. One time we got a flight the night before and stayed by the airport and picked the car up about 8:00am rather than noon reserved for, no problem. In May, I had it reserved for noon and picked up about 2:15, again no problem. One of the first times I used them I had counter service and asked about this and they said the reservation does not cancel until midnight.