The exception to this is that some locations will print out of the contract in advance for Gold members (at non-UC locations). When they do this they normally add a buffer to allow for you arriving late, but it's maybe 30-60 minutes not more.

If you pickup 2 hours late, the original time (+ buffer) will stand as the pickup time unless you explicitly ask them to update it.

Any time the contract is printed at pickup time, including UC, it'll be the actual pickup time and the allowed return time will be multiples of 24 hours later, plus the 29 minute grace period although the contract will still show the time you selected for return.

There used to be some exceptions to this, such as weekend rates that required the car to be returned by midday Monday and returning it later would cause the rate to be recalculated at a higher per-day rate (even if the number of days didn't increase), but these seem to have disappeared in recent years.