California Code, Revenue and Taxation Code - RTC § 2513

If an application, tax statement or claim for credit or refund required by law to be filed with a taxing agency on or before a specified date is filed with the taxing agency through the United States mail, properly addressed with the required postage prepaid, it shall be deemed filed on the date shown by the post office cancellation mark stamped on the envelope containing it, or on the date it was mailed if proof satisfactory to the tax collector establishes that the mailing occurred on an earlier date.

If an application, tax statement or claim for credit or refund required by law to be filed with the taxing agency on or before a specified time on a specified date is sent through the United States mail, properly addressed with the required postage prepaid, and the cancellation mark is placed on the envelope after it is deposited in the mail: