Can't speak for others, but Avis is a 30 min grace from:
1. Time out the gate if they scan you at the gate
2. Time on the contract if no gate scan. They usually set this for 45 mins after scheduled arrival time and don't update it. However, it can be station/employee specific. If you swap cars, or are more than an hour away from scheduled time, you're more likely to have the time manually entered. However, I've played the game of setting the time 4+ hours later to keep it within a 1 day increment. Sometimes works, sometimes doesn't.

Also, usually, 1 hour is ~1/2 of the daily rate. After that, it's a full day.