If you entered your flight number and expected arrival time to your rental record, they are supposed to wait for you. In my experience, most of the time they do. However, once, the counter agent went by the expected arrival time and when I didn't show didn't bother to check the flight number to see if the flight was delayed - they closed up and went home. Luckily another car rental company was still open and helped me out.

Two weeks out, you should be able to edit your arrival time?

Also, on your rental confirmation should be the local office direct phone number. Wait thru all the prompts and usually "Press 9" gets you to local counter. At the bigger airports, the phone usually rings and rings and sometimes get option to leave a message - so do, with your confirmation number and details and maybe even if they don't call you back they might at least get your info.

Good luck