They say that it opens at 10:30. As long as clearing the ship from the previous voyage (the US customs clearance) went smoothly, they start allowing vehicles in at 10:00. However...if the customs clearance was delayed (we had this happen on one of our 9 cruises so far), it means that the passengers from the previous cruise didn't get off the ship till "late" (like 9 am) and they may not be ready to allow cars in until a little after 10.
The point is that they don't want arriving passengers at the port until they are ready for us....usually 10, with counters opening at 10:30, but you can't be sure they will allow cars in until 10:30.