California Code, Government Code - GOV § 7522.32

(a) Final compensation shall mean the highest average annual pensionable compensation earned by the member during a period of at least 36 consecutive months, or at least three consecutive school years if applicable, immediately preceding his or her retirement or last separation from service if earlier, or during any other period of at least 36 consecutive months, or at least three consecutive school years if applicable, during the member's applicable service that the member designates on the application for retirement.

(b) On or after January 1, 2013, an employer shall not modify a benefit plan to permit a calculation of final compensation on a basis of less than the average annual compensation earned by the member during a consecutive 36-month period, or three school years if applicable, for members who have been subject to at least a 36-month or three-school-year calculation prior to that date.