I would think they will start sign ups as soon as the park opens 8:30. The JTA times are listed as 9:20, 10:10, 11:00, 11:50 and so on. Keep in mind the 8:30 test is only a test and may not be happening during your trip.
Good advise, I will double check when we get there and change the ADR back to 8:00 if need be. I think we'll be able to do that with no problem because they had lots of different times available when I called today. All the crowd calendars show crowd estimates at a "1" for Feb. 1st