State Tax Day - Current, S.3, California—Multiple Taxes: Guidance Issued on Changes to USPS Postmark Policy, (Mar 23, 2026)

The U.S. Postal Service (USPS) recently updated its guidance on what a postmark date represents and the change may affect the timely filing of tax returns and payments.

Postmark Date May Be Later Than Date Item Was Mailed

Under the updated Domestic Mail Manual, the postmark date on most mail now reflects the date the item is first processed at a postal facility, not necessarily the date it was placed in a mailbox or collected by a carrier. This means that the postmark date may be later than the date a taxpayer mailed their tax return, payment, petition, or claim for refund.

Avoid Late Filing Penalties and Interest

California law generally considers a document or payment timely if it is postmarked on or before the due date. As a result, a later postmark could result in penalties and interest for late filing. To avoid that, taxpayers are encouraged to:

• file and pay online;

• mail early by sending returns and payments several days before the deadline; and

• visit the post office if mailing close to the due date and request a manual postmark to ensure the date reflects when the USPS accepted the item and/or obtain a certificate of mailing or use registered or certified mail for proof of mailing.

Publication 388, Tax Information Bulletin, California Department of Tax and Fee Administration, March 2026