Section 17. The time and manner of excusing members of police departments from duty in any town subject to any of the six preceding sections shall be determined by the chief, superintendent or other officer or board at the head of the police department. A member so excused shall be exempt from duty and from attendance at a police station or other place, but otherwise shall be subject to all laws, rules and regulations relating to members of the department to which he belongs. The chief, superintendent or other officer or board at the head of the police department of any such town may, in case of any public emergency, or of any unusual demand for the services of the police in that town, prevent any member of the department from taking the day off at the time when he is entitled thereto, or at the time assigned therefor, provided that such day off shall be granted to him as soon thereafter as is practicable. In no case shall the number of such days off be less than twelve in each year in a town subject to section fourteen, or twenty-four in each year in a town subject to section fifteen, or forty-five in each year in a town subject to section sixteen, or fifty-two in each year in a town subject to section sixteen A, or sixty in each year in a town subject to section sixteen B, or one hundred and four in each year in a town subject to section sixteen C, and they shall be in addition to any annual vacation now or hereafter allowed to members of the said departments, and such annual vacation shall not be diminished on account thereof.