It seems there isn't a definite answer to this. When we were there a few weeks ago, everyone was being allowed to enter the park,. Non-resort guests were held in the hub. As resort guests, we were scanned and permitted to line up outside Tomorrowland before 8:30 and enter Tomorrowland at 8:30. But the ability to book a
DAS return time didn't become active until at least sometime after 8:45 when we boarded Space Mountain. It didn't work before us boarding but was active by the time we were exiting Space. I asked a CM about it and she said it varies and sometimes it's not active until 9:00. It probably does have to do with when standby times are posted.