My experience in the USA is that you can usually collect a car early, but there is no guarantee of this - if you want to be sure best call. It's also worth pointing out that your hire will start from when you collect the car, even if this is before the time you booked. So a car collected at 10am is due back at 10am.
They are very strict about late returns. Most commonly a 30 minute grace period is provided, and returns after that are charged the extra day. With some companies it is possible to extend a booking at an hourly rate for some hours (most commonly up to 2) but if you don't call up and extend ahead of the return time, you will be charged the extra day.
I returned a car 45 mins late in SFO and was charged the extra day, and there was nothing I could do about it.