From the California Department of Industrial Relations:
Eight hours of labor constitutes a day’s work, and employment beyond eight hours in any workday or more than six days in any workweek is permissible provided the employee is compensated for the overtime…
So while AstroChuck’s answer is actually more accurate (there is no legal mandate to work in most situations) from what I could see your employer could ask you to work the whole day, provided you get adequate breaks and overtime. I know here in Ontario, Canada we have certain rules about maximum hours worked per certain period, I don’t know if the same is true where you are.