Gov. Code, § 21008

Time during which a member is excused from performance of his or her duties, whether or not he or she is required to perform any portion of those duties during that time, and for which he or she receives compensation, but in an amount less than the full compensation earnable by him or her while performing his or her duties when not so excused, such as sabbatical leave, shall be credited as service in the proportion that the compensation paid to the member bears to the full compensation that would be earnable by him or her while performing his or her duties on a full-time basis. However, the member shall receive full-time credit for the time if after returning to the employment from which he or she was excused and at any time prior to retirement he or she elects to, and does, 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.