California Code, Government Code - GOV § 31462

(a) “Final compensation” means the average annual compensation earnable by a member during any three years elected by a member at or before the time the member files an application for retirement, or, if the member fails to elect, during the three years immediately preceding their retirement.  If a member has less than three years of service, their final compensation shall be determined by dividing their total compensation earnable by the number of months of service credited to them and multiplying by 12.

(b) This section shall not apply to a member who is subject to the California Public Employees' Pension Reform Act of 2013 for all or any portion of their membership in the county retirement system.