Employment Law Questions? Ask an Employment Lawyer.
The employer can ask you to work any hours that they want, provided that you are paid properly for your time. If you are salaried exempt, then you don't get extra income. If you are hourly, you get extra income.
What you were hired for is only mandatory if you signed a contract of employment that specifically stated that you can only be terminated for cause AND that states that the terms of your employment cannot be altered without your consent.