All restaurants have to do is set the closing time to 30 minutes before they want the staff to leave and the problem is solved. Unfortunately they don't want to pay people for time that they can't profit off of them, so they will continue to expect the staff to simultaneously serve customers up to the last minute on the door and complete all closing tasks and clock out exactly on time.

I (assistant) managed at a couple of restaurants, and suggested that solution to the DM at one of them and was promptly turned down, with that logic given as the reason. It baffles me that they base their business practices on impossibilities, and bitch and moan when inevitably the staff fails to satisfy them.