Cal. Government Code § 20638

The highest annual average compensation during any consecutive 12- or 36-month period of employment as a member of a county retirement system shall be considered compensation earnable or pensionable compensation pursuant to Section 7522.34, whichever is applicable, by a member of this system for purposes of computing final compensation for the member provided: (a) (1) Entry into employment in which the person became a member in one system occurred on or after October 1, 1957, and within 90 days of discontinuance of employment as a member of the other system. (2) This subdivision shall not deny the benefit of this section to any person retiring after October 1, 1963, who entered membership prior to October 1, 1957, if the person entered the employment in which they became a member within 90 days of termination of employment in which they were a member of the other system, and they became a member within seven months of entry into employment, or, if an employee of a district as defined in Section 31468, became a member at the time the district was included in a county retirement system. (b) The member retires concurrently under both systems and is credited with the period of service under the county system at the time of retirement.