Gov. Code, § 55819

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 pursuant to Chapter 9 (commencing with Section 860) of Title 10 of Part 2 of the Code of Civil Procedure within six months from the date of the election at which the ordinance is approved or, in the case of any ordinance approved prior to the effective date of this section, six months from the effective date of this section. Otherwise, the bonds and all proceedings in relation thereto, including the adoption and approval of the ordinance and the retail transactions and use tax provided for therein, shall be held to be valid and in every respect legal and incontestable.