California Code, Government Code - GOV § 31863

Except as otherwise provided in Section 31864, any member within 30 days after this article becomes operative in the county, or within 30 days after entry into service, or during the calendar months of November or December of any year, may elect to make contributions for, and participate in, the benefits provided for by this article. An election made more than 30 days after this article becomes operative in this county and more than 30 days after entrance into service shall become operative the following January first.