My husband works for a general engineering contractor. For the past month and a half he has been working nights instead of days (ie. 7pm-6am). The payroll department is calculating the wages based on this formula:12am-6am & 7pm-midnight in order to avoid paying overtime wages. In actuality, he works 11 hours straight through, not 6 hours and then another 5 hours as they pay it. Is this legal? I thought that pay and overtime was based on the number of hours per shift not by the calendar day.

Overtime is based on a workday and workweek. The time that starts a workweek is the same time that starts each workday.
I assume his workday starts at 2400 so he is working
12am-6am = 6hrs and
7pm-midnight = 5hrs for a totall of 8 regular and 3 at 1.5 for the workday.
He should also be intitled to 2 meal periods and 3 paid rest periods per shift. There is a 1hr penalty for each day he he doesn't receive them.
If the employer refuses to pay the OT you can have him file a wage claim with the DLSE. The site below will get you started.

http://www.dir.ca.gov/dlse/HowToFileWageClaim.htm