California Code, Government Code - GOV § 21027

(a) “Public service” with respect to a local member who retired pursuant to this part before the effective date of the election of their employer to be subject to Section 21024 also means active service with the Armed Forces of the United States or the Merchant Marine of the United States, including time during any period of rehabilitation afforded by the United States government other than a period of rehabilitation for purely educational purposes, and for six months thereafter prior to the person's first employment by the employer under this section in which the person was a member.

(b) Any retired person electing to receive credit for that public service shall make the contributions as specified in Sections 21050 and 21052. However, any eligible member who requests costing of service credit between January 1, 2001, and December 31, 2003, may, instead of making those contributions, make the payment calculated under this article as it read on December 31, 2000, which payment shall be made in the manner described in Section 21050.

(c) The public service shall not include military service in any period for which credit is otherwise given under this article or Article 4 (commencing with Section 20990), or to the extent that total credit under this section would exceed four years.

(d) Notwithstanding Section 21034, a retired person may select which of two or more periods of service entitles the retired person to receive public service under this section.

(e) This section shall apply to a retired person only if that person retired immediately following service as a local member, pursuant to this part, and before the effective date of the election by their employer to be subject to Section 21024.

(f) The retirement allowance of a retired person who elects to receive service credit pursuant to this section shall be increased only with respect to the allowance payable on and after the effective date of the election.