29 CFR § 794.138 - Workweek unit in applying the exemption.

(a) As is true generally with respect to provisions of the Act concerning compensation for overtime hours of work (see §§ 778.100 through 778.105 of this chapter, Overnight Transportation Co. v. Missel, 316 U.S. 572), the unit of time to be used in determining the application of all provisions of the section 7(b)(3) exemption to an employee is the workweek. As defined in § 778.105 of this chapter, an employee's workweek is a fixed and regularly recurring period of 168 hours—seven consecutive 24-hour periods. It may begin at any hour of any day set by the employer and need not coincide with the calendar week. Once the workweek has been set it commences each succeeding week on the same day and at the same hour. Changing the workweek for the purpose of escaping the requirements of the Act is not permitted.

(b) By its terms (§ 794.100), section 7(b)(3) exempts an employer from any statutory responsibility he might otherwise have for a violation of section 7(a) of the Act “by employing any employee for a workweek in excess of that specified in such subsection” without paying the overtime compensation prescribed therein, “if such employee is so employed * * * by an * * * enterprise” qualifying under section 7(b)(3) for application of its provisions to such employment and if such employee receives the compensation which section 7(b)(3) requires. Accordingly, for section 7(b)(3) to apply to any workweek when an employee is employed for hours in excess of those specified in section 7(a), it must be established that in such workweek he is employed by his employer in the exempt activities of an enterprise described in section 7(b)(3) and that the compensation received by him for his work in such workweek satisfies the special pay requirements of section 7(b)(3).