California Code, Code of Civil Procedure - CCP § 349.1

The validity of any acts or proceedings taken under color of law for the formation, organization, incorporation, dissolution, consolidation, change of organization or reorganization of, or for any change in the territorial boundaries of, any city, county, city and county, special district, public corporation or other public entity, or improvement district within any of the foregoing, shall not be contested in any action unless such action shall have been brought within six months from the date of completion of said acts or proceedings. Unless an action is commenced within said period all said acts or proceedings shall be held valid and in every respect legal and incontestable.

This section shall not amend or repeal any existing statute prescribing a shorter period of limitation than that specified herein.