Isaac chose 9pm EST because that's a convenient time for him to be able to do stuff. He doesn't necessarily have to be online and active when auctions end though. As long as the deadline is announced ahead of time, it's a simple matter of checking timestamps on posts to see who got bids in on time.

I would recommend selecting a day for the end of each Outcry auction and then setting the deadline for that date at (1d24) : (1d60) : (1d60) RAFO time.