Certain time. More often than not, the owner wants the employees clocking out right at close. They get upset if they close at ten but their "greedy" employee doesn't clock out until 10:15 or 10:30... Despite there being a legitimate 30 minutes or more of closing processes to do.
Employees are forced to either start closing earlier than the posted time, and risk the anger of a customer who may or may not show up, or stay fully open until the posted time and face the certain wrath of their manager yelling at them for going over their scheduled hours.

Employees will avoid the guaranteed yelling over the hypothetical upset customer every time.

In any case this is 100% a management issue.