The term “on duty” is a unique concept applicable only to employees for whom exemption is claimed. This term means the period of time during which an employee is considered to be on duty for purposes of determining compensable hours. It may be a scheduled or unscheduled period. Such periods include “shifts” assigned to employees often days in advance of the performance of the work. Scheduled periods also include time spent in work outside the “shift” which the public agency employer assigns. For example, a police officer may be assigned to crowd control during a parade or other special event outside of his or her shift.