Cal. Pub. Util. Code § 150213

Any action or proceeding wherein the validity of the adoption of the retail transactions and use tax ordinance provided for in this chapter or the issuance of any bonds thereunder or any of the proceedings in relation thereto is contested, questioned, or denied, shall be commenced within six months from the date of the election at which the ordinance is approved; otherwise, the bonds and all proceedings in relation thereto, including the adoption and approval of the ordinance, shall be held to be valid and in every respect legal and incontestable.