Do restaurants only pay for business hours, and not for set-up/clean-up?

I work in a restaurant in California, and I recently received my paycheck. The hours did not match up to what I had worked, and I asked the manager about it. She told me that the company only paid for business hours, even though I'm asked to get there 30 minutes before to help set up. The restaurant closes at 10pm, but several times a week, there will be customers who show up at 9:40pm or later and sit down to eat. I discussed this with my manager, and she told me that I must stay and serve the customer until they leave, but I will not get paid for the time I have to stay there after 10pm. After the customer leaves, I must still help with the clean-up, so sometimes I'm at work past 11pm. Is my employer required to pay me for that extra time?