California Code, Revenue and Taxation Code - RTC § 21027

(a)(1) For purposes of Part 10 (commencing with Section 17001), Part 10.2 (commencing with Section 18401), Part 11 (commencing with Section 23001), or this part or any other law that is applicable to the mailing of any returns, payments, or any other items required to be filed under Part 10 (commencing with Section 17001), Part 10.2 (commencing with Section 18401), Part 11 (commencing with Section 23001), or this part, any reference in Section 11003 of the Government Code to the United States mail shall be treated as including a reference to any designated delivery service, and any reference in that section to a post office cancellation mark shall be treated as including a reference to any date recorded electronically by a designated delivery service, kept in the regular course of the designated delivery service's business, or marks on the cover in which any item is to be delivered to the board that indicate the date on which the item was given to the designated delivery service for delivery.

(2) For purposes of this section, “designated delivery service” means any delivery service provided by a trade or business if that service is designated by the Secretary of the Treasury under the authority of Section 7502(f) of the Internal Revenue Code,  1 as amended by Public Law 104-168. 2