Notwithstanding Section 31558 and regardless of age at entry into the system in counties subject to the provisions of Section 31676.1 and in counties electing pursuant to Section 31695.1, a person shall become a safety member on the first day of the calendar month following his entrance into service in a position the principal duties of which are defined in Sections 31470.2, 31470.4 or 31470.6, if such first day of the calendar month is within 90 days, or six months if Section 31840.4 applies of last rendering active police or fire suppression or lifeguard work as a member of the Public Employees’ Retirement System or a retirement system established under this chapter in another county, the State Teachers’ Retirement System, or a retirement system of any other public agency of the state that has established reciprocity with the Public Employees’ Retirement System subject to the conditions of Section 31840.2, and the person retains his membership in such other system.