Rev. & Tax. Code, § 2512

(a) If a remittance to cover a payment required by law to be made to a taxing agency prior to a specified date and hour is (a) deposited in the United States mail in a sealed envelope, properly addressed with the required postage prepaid, or (b) deposited for shipment with an independent delivery service that is an Internal Revenue Service designated delivery service or has been approved by the tax collector, in a sealed envelope or package, properly addressed with the required fee prepaid, delivery of which shall not be later than 5 p.m. on the next business day after the effective delinquent date, the remittance shall be deemed received on the date shown by the post office cancellation mark stamped upon the envelope containing the remittance, or the independent delivery service shipment date shown on the packing slip or air bill attached to the outside of the envelope or package containing the remittance, or, in the absence of any of the aforementioned dates, on the date received by the county treasurer-tax collector, or on the date it was mailed if proof satisfactory to the tax collector establishes that the mailing occurred on an earlier date. The taxing agency is not required to accept a payment actually received in the mail if it is received more than 30 days after the date and time set by law for the payment.

(b) If a remittance to cover a payment, required by law to be made to a taxing agency prior to a specified date and hour, is made by an electronic payment option, such as wire transfer, telephoned credit card, or electronic internet means, the remittance shall be deemed received on the date the transaction was completed by the taxpayer, if the remittance was made on the taxing agency’s authorized internet website or via the taxing agency’s authorized telephone number. Proof of completion of the transaction in the form of a confirmation number or other convincing evidence shall be presented by the taxpayer to the satisfaction of the tax collector. This subdivision does not apply to payments by electronic fund transfer as provided in Sections 2503.1 and 2503.2.

(c) This section does not, for purposes of applying subdivision (a) of Section 3707, apply to a remittance sent by mail, by independent delivery service, or by electronic payment option for the redemption of tax-defaulted property.