I cancelled an award ticket reservation recently (not Plat yet) at about 2315 in SLC, where I was located / the origin of the ticket. The rep claimed it's based on Atlanta time, but offered to refund it as a 'one time courtesy' and said that in the future I needed to cancel by midnight EST.

Incidentally, the miles were initially redeposited two days later, but then were pulled back out the following day, and the fees have not been refunded. I haven't called back in to get it handled yet, but I don't anticipate any trouble getting it sorted.