California Code, Government Code - GOV § 31878

(a) This article may be adopted by any county by ordinance.

(b) The ordinance shall specify a date as of which Article 16.5 shall be deemed applicable.  The applicable date may be the first day of any prior year.  If no applicable date is specified, Article 16.5 shall be deemed applicable as of the first day of the year preceding the year of retirement of the earliest retired member who is currently being paid a retirement allowance at the time the ordinance is adopted.

(c) The ordinance shall specify a date as of which the adjustments made pursuant to this article shall be effective.  If no effective date is specified, the adjustments shall be made in allowances payable for time commencing on the first day of the month following the adoption of the ordinance.