Cal. Elections Code § 3020

(a) All vote by mail ballots cast under this division shall be received by the elections official from whom they were obtained or by the precinct board no later than the close of the polls on election day. (b) Notwithstanding subdivision (a), any vote by mail ballot cast under this division shall be timely cast if it is received by the voter’s elections official via the United States Postal Service or a bona fide private mail delivery company no later than seven days after election day and either of the following is satisfied: (1) The ballot is postmarked on or before election day or is time stamped or date stamped by a bona fide private mail delivery company on or before election day, or it is otherwise indicated by the United States Postal Service or a bona fide private mail delivery company that the ballot was mailed on or before election day. (2) If the ballot has no postmark, a postmark with no date, or an illegible postmark, and no other information is available from the United States Postal Service or the bona fide private mail delivery company to indicate the date on which the ballot was mailed, the vote by mail ballot identification envelope is date stamped by the elections official upon receipt of the vote by mail ballot from the United States Postal Service or a bona fide private mail delivery company, and is signed and dated pursuant to Section 3011 on or before election day. (c) For purposes of this section, “bona fide private mail delivery company” means a courier service that is in the regular business of accepting a mail item, package, or parcel for the purpose of delivery to a person or entity whose address is specified on the item.