Working off the clock is a lot different than reporting for your normal hours without pay. When I was an RN, I always showed up early and reviewed my records and got my meds ready and so forth before my shift started in order to have a smoother day. Sure, I get that. But if my supervisor had called me up and and said, "we need you to work 5 shifts this week, but we can't pay you," I would have laughed at her and started looking for another job.
It's a little different when you're working for the Government and the Government tells you to show up.