29 CFR 778.103: The workweek as the basis for applying section 7(a).

If in any workweek an employee is covered by the Act and is not exempt from its overtime pay requirements, the employer must total all the hours worked by the employee for him in that workweek (even though two or more unrelated job assignments may have been performed), and pay overtime compensation for each hour worked in excess of the maximum hours applicable under section 7(a) of the Act. In the case of an employee employed jointly by two or more employers (see part 791 of this chapter), all hours worked by the employee for such employers during the workweek must be totaled in determining the number of hours to be compensated in accordance with section 7(a). The principles for determining what hours are hours worked within the meaning of the Act are discussed in part 785 of this chapter.