Your best bet is to call National and ask before your trip as well as during your trip. My experience with Hertz and Avis is a 59 minute grace; however, you never know. Then again, Avis and Hertz have both managed to sneak in gas refueling charges that I didn't notice until I got home. Your second safest bet is to wait at the airport an extra 30 minutes to pick up your car (and remember the clock starts ticking once you show up at the counter). On a 10-day rental, hopefully, that won't be such an inconvenience.