They can assign a report time as early as the start of your SC window, with the knowledge that you aren't required to show up at the airport until the 2 hour point. The PWA wording says a pilot who invokes non-contactable cannot be assigned a rotation that reports earlier than 2 hours into the SC window.

Scheduling also often gets confused that you go back onto the "promply available" leash once you check your schedule. So say you've got 1200 SC. You check your schedule at 1300 and no asignments have been given. Then scheduling calls at 1330 and assigns a 1345 report. They still expect you to be there by 1400 (immediately available leash) even though youv'e reverted to promptly available.