Cal. Government Code § 31641.56

A member who returns to county service after having been on deferred status while rendering county service in a position excluded from membership, shall have the right to receive credit in the retirement association for all or any part of the county service he or she rendered during the time he or she was on deferred status if he or she elects to pay, and thereafter pays, in accordance with the election, and prior to retirement, into the retirement fund an amount equal to the contributions he or she would have made to the retirement fund if he or she had not been on deferred status during the time for which he or she has elected to receive credit, computed by applying the contribution rate first applicable to him or her upon returning to county service to the aggregate compensation he or she actually received during the time he or she was in county service and for which he or she has elected to receive credit, together with regular interest on that amount from the time he or she was on deferred status until completion of payment. The payment may be made by lump sum or by installment payments over a period not to exceed the length of time for which the member has elected to receive credit, in the manner and at the times as the board may by rule prescribe. No member shall receive credit under this section for any service for which he or she has not completed payment pursuant to this section before the effective date of his or her retirement. A member who has elected to make payment in installments may complete payment by lump sum at any time prior to the effective date of his or her retirement. Any sums paid by a member pursuant to this section shall be considered to be administered as normal contributions by the member. A member who elects to receive credit for only a part of that county service shall elect that county service latest in time and may not receive credit for any portion of that county service for which he or she does not elect to receive credit. This section shall not be operative in any county until the time that the board of supervisors shall, by resolution adopted by majority vote, make the provisions of this section applicable in the county.