Cal. Gov. Code § 31680.4
Notwithstanding any other provision of law, a member retired for service and reemployed in a county or district under this chapter shall become again an active member of the retirement association upon (a) his or her application to the board for reinstatement, (b) the determination of the board, based upon medical examination, that he or she is not incapacitated for the duties assigned to him or her; and (c) meeting the conditions for membership in Article 4 (commencing with Section 31550) are met.
For the purposes of this section, the effective date of the member's reinstatement to active membership shall be the first day of the month following the date of reemployment.
Except as permitted in Section 31680.2 or 31680.3, the retirement allowance of the member shall be canceled on the effective date of the member's reemployment and shall be resumed only upon the subsequent termination of the member from employment.
This section shall not be operative in any county until the board of supervisors, by resolution adopted by a majority vote, makes this section and Section 31680.5 operative in that county.
Ca. Gov. Code § 31680.4