(a) Except as provided in subsection (b), no employer shall be subject to any liability or punishment under the Fair Labor Standards Act of 1938, as amended, * * * on account of the failure of such employer to pay an employee minimum wages, or to pay an employee overtime compensation, for or on account of any of the following activities of such employee engaged in on or after the date of the enactment of this Act:

(1) Walking, riding, or traveling to and from the actual place of performance of the principal activity or activities which such employee is employed to perform, and

(2) Activities which are preliminary to or postliminary to said principal activity or activities

which occur either prior to the time on any particular workday at which such employee commences, or subsequent to the time on any particular workday at which he ceases, such principal activity or activities. For purposes of this subsection, the use of an employer's vehicle for travel by an employee and activities performed by an employee which are incidental to the use of such vehicle for commuting shall not be considered part of the employee's principal activities if the use of such vehicle for travel is within the normal commuting area for the employer's business or establishment and the use of the employer's vehicle is subject to an agreement on the part of the employer and the employee or representative of such employee.

(b) Notwithstanding the provisions of subsection (a) which relieve an employer from liability and punishment with respect to an activity, the employer shall not be so relieved if such activity is compensable by either:

(1) An express provision of a written or nonwritten contract in effect, at the time of such activity, between such employee, his agent, or collective-bargaining representative and his employer; or

(2) A custom or practice in effect, at the time of such activity, at the establishment or other place where such employee is employed, covering such activity, not inconsistent with a written or nonwritten contract, in effect at the time of such activity, between such employee, his agent, or collective-bargaining representative and his employer.

(c) For the purpose of subsection (b), an activity shall be considered as compensable under such contract provision or such custom or practice only when it is engaged in during the portion of the day with respect to which it is so made compensable.

(d) In the application of the minimum wage and overtime compensation provisions of the Fair Labor Standards Act of 1938, as amended, * * * in determining the time for which an employer employs an employee with respect to walking, riding, traveling, or other preliminary or postliminary activities described in subsection (a) of this section, there shall be counted all that time, but only that time, during which the employee engages in any such activity which is compensable within the meaning of subsections (b) and (c) of this section.