It should be based off the time zone in ATL. For some odd reason, when I did this, it was giving me issues (I live in the Pacific time zone) until I switched the time on my laptop to match ATL (Eastern time) while I booked the ticket. Not sure why - it could have just been a coincidence that the schedule update finally finished loading but when I switched the time on the computer to eastern time temporarily to book the ticket, it magically worked.