Let's say you have 3-day rental with a 3PM pickup and return. You show up early and check out the car at 2PM. You will almost certainly see that your rental contract shows a 3 day and 1 hour rental with a charge for the extra hour.

Now what will happen is if you return the car at or before 2PM, then you will only be charged for 3 days and not 3 days and 1 hour. Technically you have a 29 minute grace period, so you'd have until 2:29PM before that extra hour kicked in.

The bottom line is that rentals are priced on a 24-hour day. The times you reserve for are almost immaterial in many cases. If you go over the number of days that you originally reserved for (days being defined as a 24-hour period), then you will likely pay a higher rate.