If a remittance to cover a payment of interest or interest and principal to be made to the treasurer prior to a certain date and hour or prior to a certain date before being delinquent is sent through the United States mail, properly addressed with postage prepaid, and the cancellation mark is placed on the evelope after it is deposited in the mail:
(a)
Where the cancellation mark shows both date and time, the remittance shall be deemed received on the date shown by the cancellation mark and by the time specified by law for that date.(b)
Where the cancellation mark shows only the date, the remittance shall be deemed received within the time and date specified when the cancellation mark bears a date on or before which payment is required. The treasurer need not accept such a payment if it is received more than 10 days after the date and time set by law for the payment. This section shall not apply if the letter containing the payment was incorrectly addressed.
Source:
Section 6441.1, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=SHC§ionNum=6441.1..