If I picked up late, can I also return later?
 

Say I make a one day rental, 12pm to 12pm the next day.

If I picked up at 1:30pm, can I return at 1:30pm the next day without additional charges?

Yes. Charges are in 24hr blocks

Seconded, what the above poster said. Also, most rental car companies have 29 min late return grace period, which you can calculate depending on your actual pick up time. This way you know precisely when late charges [next day hourly-daily charges] will kick in. I know Hertz prints hourly/daily charges on the contract so you know what you will pay if late.

IME the grace period tends to apply only to rentals in the USA and airport locations use the time that you leave the gate as the start of your rental contract. Also, if you're too late (two hours?), the reservation might be cancelled and they could generate a new contract at the current rates if you stand in line and cars are available.

OP seems to be renting from National, but we don't know the country and whether it's an airport or city location. Also, OP has status with National which might make a difference.

Read your contracts carefully. Recently National has not been giving me paper contracts even when I requested one. Make sure you double check your return time on the contract. Many rental car companies keep the reserved return time regardless of the pick up time. There have been times the car company refused to extend the return time even when I picked up late. I don't believe it was National. I usually make reservation for the time I want to return and if I am 30 minutes early, they generally do not hassle me. If I pickup more then 30 minutes early, they may tack on extra charges.

If you pick up more than 30 minutes early then the computer system assumes you are still dropping off at the originally scheduled return time so you'll see the contract amount increase. This is nothing to worry about. Just return on your scheduled return day at or before (actually there's usually a 29-minute grace period) the time you originally picked up and you'll be charged your originally contracted amount. This often confuses people but it's just the counter system making certain assumptions and is nothing to worry about.

Thank you all.

It appears National isn't one of these companies. I was renting in the US; maybe foreign locations operate differently.

Similar question:
if I make a reservation for 3 days but then change my flight and return at exactly 48 hours plus 29 minutes, will my rate be auto adjusted for 2 days instead.
Suppose I used a coupon ($20 off 3 day rental) that had a three day minimum...will that drop off automatically?

it isn’t a big deal either way but it would be nice to save a few bucks.

It depends on the rental company but often the answer is yes--if you are on a daily rate and you have a 3-day rental but return after 2 days then you will only pay for 2 days. In some cases there can be an 'early return fee" but more often than not, this won't apply.

If you have a coupon that requires a minimum of a 3-day rental and the rental no longer meets the length restriction, then yes, the coupon will usually fall off and won't be applied.