The rate of extra compensation for authorized overtime services performed at night on any week day is hereby fixed at one half the gross daily rate of regular pay of the employee who performs the services for each 2 hours of compensable time, any fraction of 2 hours amounting to at least one hour to be counted as 2 hours. In computing the amount earned, each 2 hours is the time period for the purpose of computation, at least one hour means the minimum service in each period for which extra pay may be granted. If service continues beyond a 2 hour period, it must extend for at least one hour into the following 2 hour period to be entitled to extra pay for the second period. When the overtime extends beyond 5 p.m., payment of extra compensation from 5 p.m. for services consisting of at least one hour is authorized, even though such services may not actually begin until 7 p.m., 9 p.m., or later: Provided, That the officer rendering the service remained on duty from 5 p.m., in which case the time between 5 p.m., and the time of beginning the actual service shall be computed as waiting time; and where the actual services begin as late as 9 p.m., there should be an affirmative statement that the officer was required to remain on duty between 5 p.m. and 9 p.m., if a charge for waiting time is made. The maximum amount of extra compensation which may be paid an employee for services during one night shall not exceed two and one-half times the gross daily rate of his regular pay.