Cal. Revenue and Taxation Code § 166

(a) Whenever a taxpayer is required to file any statement, affidavit, application, or any other paper or document with a taxing agency by a specified time on a specified date, such filing shall be deemed to be within the specified period if it is sent by United States mail, properly addressed with postage prepaid, and bears a post office cancellation mark of the specified date, or earlier within the specified period, stamped on the envelope, or on itself, or if proof satisfactory to the agency establishes that the mailing occurred on the specified date, or earlier within the specified period. (b) The provisions of this section shall supersede any contrary special provision of this division unless such special provision specifically provides that this section shall not be applicable. (c) The provisions of this section are applicable to any filing required to be made by ordinance, rule, or regulation of a taxing agency. (d) Any statement or affidavit made by a taxpayer asserting such a timely filing must be made within one year of the deadline applicable to the original filing; provided, however, that this subsection shall not apply to any statement or affidavit asserting the timely filing of a property statement or to any statement made by the taxpayer in connection with an escape assessment imposed pursuant to Section 531. (e) It is the intent of the Legislature that this section be liberally construed in favor of the taxpayer and be applicable to all filings relating to property taxation which are required to be made by a taxpayer by a specified time on a specified date.