They say to arrive 15-20 minutes before your ADR time, which pretty much puts you entering AK right at 9, though may be able to enter 15-20 minutes ahead of time, it can't hurt to ask.
They might let you in the park a few minutes early since you will have to take time to walk to TH. I would get there about 8:45am and ask. Worst case: ya wait 15 minutes to get in the gate.