An employee subject to this subsection whose regular work schedule includes an 8-hour period of service a part of which is on Sunday is entitled to additional pay at the rate of 25 percent of his hourly rate of basic pay for each hour of work performed during that 8-hour period of service. For employees serving outside the United States in areas where Sunday is a routine workday and another day of the week is officially recognized as the day of rest and worship, the Secretary of State may designate the officially recognized day of rest and worship as the day with respect to which the preceding sentence shall apply instead of Sunday. Time spent in a travel status away from the official duty station of an employee subject to this subsection is not hours of work unless the travel (i) involves the performance of work while traveling, (ii) is incident to travel that involves the performance of work while traveling, (iii) is carried out under arduous conditions, or (iv) results from an event which could not be scheduled or controlled administratively (including travel by the employee to such event and the return of the employee from such event to the employee’s official duty station). The first and third sentences of this subsection shall not be applicable to an employee who is subject to the overtime pay provisions of section 7 of the Fair Labor Standards Act of 1938. In the case of an employee who would, were it not for the preceding sentence, be subject to the first and third sentences of this subsection, the Office of Personnel Management shall by regulation prescribe what hours shall be deemed to be hours of work and what hours of work shall be deemed to be overtime hours for the purpose of such section 7 so as to ensure that no employee receives less pay by reason of the preceding sentence.