Section 17C. If a regular police officer in the service of a city or town which has not accepted section seventeen B shall be required to be on duty for any period in excess of his regular hours of duty as from time to time established, he may be given time off equal to such period of overtime duty or, if time off cannot be given by reason of a personnel shortage or other cause, he may be paid for such period of overtime duty at such an hourly rate as may be determined by the authority in charge of the police department, which rate shall in no event be less than one and one half times the hourly rate of his regular compensation for his average weekly hours of regular duty.