An employee who performs work during a regularly scheduled 8-hour period of service which is not overtime work as defined by
section 5542(a) of this title a part of which is performed on Sunday is entitled to pay for the entire period of service at the rate of his basic pay, plus premium pay at a rate equal to 25 percent of his rate of basic pay. 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.