California Code, Government Code - GOV § 11003

If an application, tax return or claim for credit or refund required by law to be filed with the state or state agency on or before a specified date is filed with a state agency through the United States mail or through a bona fide commercial delivery service, as determined by the state or the state agency addressee, properly addressed with postage prepaid, it shall be deemed filed on the date shown by the cancellation mark stamped on the envelope containing it, or on the date it was mailed if proof satisfactory to the state agency establishes that the mailing occurred on an earlier date.

If an application, tax return or claim for credit or refund required by law to be filed with the state or state agency on or before a specified time on a specified date is sent through the United States mail or through a bona fide commercial delivery service, as determined by the state or the state agency addressee, properly addressed with postage prepaid, and the cancellation mark is placed on the envelope after it is deposited in the mail: