California Code, Government Code - GOV § 81007

When a report or statement or copies thereof required to be filed in paper format with any filing officer under this title has been sent by first-class mail or by any other guaranteed overnight delivery service addressed to the filing officer, it shall for purposes of any deadline be deemed to have been received by the filing officer on the date of the deposit in the mail or of receipt by that delivery service.  It shall be presumed until the contrary is established that any date stamped by the post office on the envelope or contained on the delivery service receipt containing the report or statement is the date it was deposited in the mail or received by the delivery service.  Mail that is not received by the filing officer shall be presumed not to have been sent unless the filer possesses a post office or delivery service receipt establishing the date of deposit and the name and address of the addressee.