In all cases where the contract is either printed at the exit gate, or checked at the gate, it is the time you go through that gate which counts as the start time.

If there is no gate, then it is the time that you are given the contract and keys. So the only ones to watch are where, as is the case at some European airports, there is a Gold Desk and they have pre-printed the contract and they simply hand it to you, as the time then will be what you put in your reservation. If there is a significant difference you make need to ask them to amend it on their system. When you return the car, it will be time that was on their system that will be used to calculate how long you had the car.

By the way, I always put a collection time of later than the flight arrival time - I usually add my best estimate of how long it will take, allowing for baggage collection etc, as sometimes they will pull your name off the Gold Board if you are say more than 2 hrs after what you put on your reservation.