stupid ?-turn in time

You have 5 minutes before through 5 minutes after the turn in time (turn-in window).
So using your example, the turn in window would begin at 12:55, and end at 1:05. If you are early, you will stand there waiting. If you are late, you will return back to your cooksite with your box.

Yes rules do differ among sanctioning bodies. For example, in GBA, a typical turn in time would be 10:30 to 10:45. Turn in times are posted exactly like that, not just 10:30.

I assumed that since he mentioned he was watching BBQ Pitmasters he was asking KCBS as I thought that was the vast majority of their coverage. So yes, please refer to the rules of the sanctioning body that you may be thinking about competing in.

To be clearer, the turn-in times for KCBS particularly are 11:55:00 to 12:04:59.

If you are not within arms' reach of the table at 12:05:00, you will more than likely be sent back with your box. Some reps may or may not give 5'-10'-15'-20' of leeway.

All of the reps we have cooked under will be out front keeping a watchful eye on the box runners...slowing them down if they see them hurrying for no reason.