Cal. Water Code § 36830.6

If a remittance to cover a payment required to be made on or before a specified date is sent through the United States mail, properly addressed with postage prepaid, it shall be determined received on the date shown by the post office cancellation mark stamped upon the envelope containing the remittance or on the date it was mailed if proof satisfactory to the tax collector establishes that the mailing occurred on an earlier date, provided, however, that the tax collector need not accept such a payment if it is received more than 30 days after the date on or before which the payment was delinquent.