California Code, Government Code - GOV § 31641.5

A member who prior to his entrance into the retirement system held a position in the county service the tenure of which was such as to exclude him from membership, or a coroner, public administrator or coroner-public administrator who was excluded because of the means by which he was compensated, shall have the right to receive credit in the retirement association for all or any part of the county service he rendered, whether interrupted or not, during the time he was in county service before becoming a member if he elects to pay, and thereafter pays, in accordance with such election, and prior to retirement, into the retirement fund an amount equal to the contributions he would have made to the retirement fund if he had been a member during the time for which he has elected to receive credit, computed by applying the contribution rate first applicable to him upon becoming a member to the aggregate compensation he actually received during the time he was in county service and for which he has elected to receive credit, together with interest at the current rate on that amount from the time he became a member until completion of payment.  Such 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 such manner and at such times as the board may by rule prescribe.  No member shall receive credit under this section for any service for which he has not completed payment pursuant to this section before the effective date of his retirement.  A member who has elected to make such payment in installments may complete payment thereof by lump sum at any time prior to the effective date of his retirement.  Any sums paid by a member pursuant to this section shall be considered to be and administered as normal contributions by the member.

Where county service prior to membership was rendered on a part-time basis, the member may receive credit for such proportion of the time he held the part-time position as the time he was actually engaged in the performance of the duties of such position bears to the time required to perform the same duties in a full-time position.

A member who elects to receive credit for only a part of such county service shall elect that county service latest in time and may not receive credit for any portion of such county service prior in time to any county service for which he does not elect to receive credit.