California Code, Elections Code - ELEC § 3020

(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.