N.Y.C. Admin. Code § 13-247: § 13-247 Retirement; selection of either twenty or twenty-five years of city-service.

a. Any person becoming a member who was not previously a member or who during his or her last previous membership in the pension fund contributed on the basis of a minimum period of retirement of twenty years of city-service, may elect, prior to the certification of his or her rate of contribution, to contribute on the basis of a minimum retirement period of twenty years of city-service, by a written election duly executed and acknowledged and filed with the board. The minimum period of retirement for such member so electing shall be twenty years of city-service, and all contributions and benefits payable by or on account of any such member shall be computed on the basis of such minimum retirement period.

b. Any person becoming a member who was not previously a member or who during his or her last previous membership in the pension fund contributed on the basis of a minimum period of retirement of twenty-five years of city-service, may elect, prior to certification of his or her rate of contribution, to contribute on the basis of a minimum retirement period of twenty-five years of city-service by a written election duly executed and acknowledged and filed with the board. The minimum period of retirement for such members so electing shall be twenty-five years of city-service, and all contributions and benefits payable by or on account of any such member shall be computed on the basis of such minimum retirement period.