A workweek is defined by and varies by employer. It is not required to be scheduled as a calendar week from Monday through Sunday. The workweek ordinarily includes all time during which an employee is necessarily required to be on the employer's premises, on duty or at a prescribed work place. Workday, in general, means the period between the time on any particular day when such employee commences his/her "principal activity" and the time on that day at which he/she ceases such principal activity or activities. The workday may therefore be longer than the employee's scheduled shift, hours, tour of duty, or production line time.