In a port at which the customary working hours begin before 8 a.m. or end after 5 p.m., the Secretary may regulate the working hours of the officers and employees referred to in section 2111 of this title so that those hours conform to the prevailing working hours of the port. However - (1) the total period for which overtime pay may be required under section 2111 of this title may not be more than 15 hours between any 2 periods of ordinary working hours on other than Sundays and holidays; (2) the length of the working day for the officers and employees involved may not be changed; and (3) the rate of overtime pay may not be changed.