It's Central Standard Time for tickets originating in the EST or CST; and then PST for Mountain/PST originating tickets. Dont ask me how I know - but I've had to refund tickets for all, and even when the agents were unsure that I was correct, they'd push the button to see if the tickets would be refunded without penalty utilizing the "24 hour guarantee" and the system accepted it. Out of all the times I've asked agents though, just for clairification, only one agent knew this policy. The rest of them are clueless and either claim they dont know, or make up their own assumptions.