As of 11:55am EST today, my account still hasn't turned 20, so I'm guessing they actually store the exact hour/minute/second that you signed up for your account too. I think I used to get home from school around 2:45pm, so I gotta keep checking.
Thx, Apparently I signed up way later in the day than I thought! I gotta wait until 11pm, lol - damn it, I wanted to show off my 20yr badge in a screenshot today. :D