29 CFR § 783.49 - Workweek unit in applying the exemptions.
The unit of time to be used in determining the application of the exemption provided by section 13(b)(6) or 13(a)(14) to an employee is the workweek. (See Overnight Transportation Co. v. Missel,316 U.S. 572; Sternberg Dredging Co. v. Walling, 158 F. 2d 678.) This is the period used in determining whether a substantial amount of non-seaman's work has been performed so as to make the exemption inapplicable. See § 783.37. A workweek is a fixed and regularly recurring interval of 7 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 of the workweek for the purpose of escaping the requirements of the Act is not permitted.