(a) The town clerk of any town which accepts sections one to twenty-eight, inclusive, by vote at any state election, as provided for in subdivision (1) of this section, shall file in the office of the public employee retirement administration commission within thirty days after such vote a certificate thereof sworn to by the election commissioners or other officers corresponding thereto. The city or town clerk of any city or town which accepts said sections one to twenty-eight, inclusive, by vote of the city council, approved by the mayor, or of the selectmen, as the case may be, as provided for in subdivision (2) of this section, shall forthwith file in the office of said commission a certificate of such vote certified by the city or town clerk. Said commission shall, within fifteen days after such filing, issue to the mayor or selectmen a certificate to the effect that the retirement system established by said sections shall become operative in such city or town on the first day of January or of July or of October, whichever occurs first, following the expiration of three months after the date of such certificate.