The unit of time to be used in determining the application of the
exemption to an employee is the workweek. (See Overnight Transportation
Co. v. Missel, 316 U.S. 572.) A workweek is a fixed and regularly
recurring interval of 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 of the
workweek for the purpose of escaping the requirements of the Act is not
permitted.