Reservations are based on 24-hour periods.
For example, if you make your reservation for 2 days 1 hour, your reservation will be considered 3 days. You will have until 2 days 24 hours to checkout before you are charged another day. If you end up parking 3 days or 26 hours, your stay will then be considered 4 days and you will be charged an extra day at checkout.
The timer starts at the time you check into the facility.