(a) For the purposes of this part, workday means the period between the commencement of the principal activities that an employee is engaged to perform on a given day, and the cessation of the principal activities for that day. All time spent by an employee in the performance of such activities is hours of work. The workday is not limited to a calendar day or any other 24-hour period.

(b) Any rest period authorized by an agency that does not exceed 20 minutes and that is within the workday shall be considered hours of work.

(c) Bona fide meal periods are not considered hours of work, except for on-duty meal periods for employees engaged in fire protection or law enforcement activities who receive compensation for overtime hours of work under 5 U.S.C. 5545(c)(1) or (2) or 5545b. However, for employees engaged in fire protection or law enforcement activities who have periods of duty of more than 24 hours, on-duty meal periods may be excluded from hours of work by agreement between the employer and the employee, except as provided in § 551.432(e) and (f).