Once a reservation is pulled (usually 24 hours before) and or otherwise system has locked it down CSR at locations cannot do anything to a contract in terms of changes. If something major needs doing it requires a manager or even higher and best that happens there is a totally new contract must be created.

System will add things of course including adjusting reservation based upon pick-up or return time as per contract versus actuality.

As others have said if reservation was for 10am collection time but vehicle is picked-up at 8am system will duly note difference. Just so you know new start time of rental is when vehicle is collected and departs location. In this case it would now be 8am. Long as ride is returned by that time on whatever day it is a 24-hour rental and that is what will be charged. If car is returned at say 10am that is well past 29 minutes grace period from 8am start time and system will charge for next full 24-hour period. National does not do one-hour rentals, regardless of vehicle being kept for less than 24 hours system will always bill by the day.

On contracts there are two separate times listed; one is for as reservation was made; other is when actual contract is created, vehicle assigned, and reservation started (as in driving off lot). Other time will be when car was due back and then actually returned. Long as time driven off lot and time returned are 24 hours (plus in case of returns grace period) you're good to go.