California Code, Government Code - GOV § 21007

“Leave of absence” also means any time during which a member is granted an approved leave for the purpose of service with a university;  college;  local, state, federal or foreign governmental agency;  or nonprofit organization, if he or she returns to employment within the terms and conditions under which the leave was granted.  A member may elect to receive service credit for that leave of absence at any time prior to retirement by making 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.  In no event shall a member receive service credit in excess of two years for each approved leave of absence.