Arriving later than reservation
 

Due to flight change, I will be arriving at National about 4 hours later than my res. Do I need to change the res or notify them, or do I just show up and go to the Aisle?

It shouldn't cancel until 24 hours have passed. Keep in mind you will still have to return at the same time as originally reserved though, you won't get an additional 4 hours on the back end even though you're late.

That won't be a problem. Flight home is not affected so it will be returned as scheduled. It's a weekly rental, so the 4 hour delay at start will not have any effect.

I've been told before that that the reservation is good for six hours from the original start time, and after that the station has the option of cancelling it if they need the car.

True, though in my experience they rarely do. I've picked up cars the next day (18 hours later) following IRROPS with no issues.

I have never found this to be the case. I've always known it to be a 24 hour clock. If you pick up at 1:45pm, then you have to return it by 1:45pm (not including the 29 minute grace period) the following day if it is a 1 day reservation.

If this has changed will someone please chime in?

Arriving later than reservation
 

I agree. Always been 24 hours from pickup. Even if you are late. Keep in mind if you don't show it's not like they bill you.

I was told 24 hours at YUL.

Flew in once late in day and staying at airport Marriott - rather than pick up rental and park in hotel garage (which was $20+) overnight, just let National keep the car and picked up in the morning (about 12 hours late) without a problem.

As an update, flight was completely cancelled so I flew to another city. Had to cancel the original res and rebook from the new city. Cost me more, of course (now a last minute rental), but so be it.