29 CFR § 778.105 - Determining the workweek.
An employee's workweek is a fixed and regularly recurring period of 168 hours - seven consecutive 24-hour periods. It need not coincide with the calendar week but may begin on any day and at any hour of the day. For purposes of computing pay due under the Fair Labor Standards Act, a single workweek may be established for a plant or other establishment as a whole or different workweeks may be established for different employees or groups of employees. Once the beginning time of an employee's workweek is established, it remains fixed regardless of the schedule of hours worked by him. The beginning of the workweek may be changed if the change is intended to be permanent and is not designed to evade the overtime requirements of the Act. The proper method of computing overtime pay in a period in which a change in the time of commencement of the workweek is made, is discussed in §§ 778.301 and 778.302.
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