Then give workforce.com's The Legal Implications of Nontraditional Workweeks a read:
The federal act’s parameters mesh reasonably well with what might be the simplest version of a less conventional workweek, which is one consisting of four 10-hour workdays. If all nonexempt employees actually stick precisely to that schedule and work no more than 10 hours in any workday, and if all four of their 10-hour workdays fall within a single workweek, then no FLSA overtime is due to any of them for such a workweek.
But this is not the case for an employee who, for instance, works four days of nine hours, 12 hours, 11 hours, and 10 hours, for a total of 42 hours in one workweek. In that situation, the employee is due two hours’ worth of FLSA overtime, even though he or she was not scheduled to work it.
With few exceptions, the FLSA does not allow employers to average or offset overtime hours worked in one workweek against non-overtime ones worked in another, or to "pay" for overtime in time off weeks or months later. This has tripped up unwary employers who adopt four-day/five-day alternating schedule.
Ordinarily, a workweek change involves work time that overlaps both the new and the old workweeks. The Labor Department will deem wages to have been paid properly for those hours if the employer: (1) assumes first that the overlapping hours were worked in the "old" workweek, then computes straight-time and overtime pay due for each of the workweeks, and then totals the sums; (2) performs the same calculation assuming instead that the overlapping hours were worked in the "new" workweek....