Read through this:

https://www.federalregister.gov/documents/2025/08/12/2025-15266/postmarks-and-postal-possession

Postmarks don't end up on all mail, because certain types of mail do not require cancellation of a stamp.

When there is a postmark, that is proof that the postal service was in possession of that piece of mail on that day. It does not prove that date was the earliest that the item came into the possession of the Postal Service, and does not foreclose the possibility that the Postal Service was in possession on an earlier date.

For the big operational change being implemented, where zip codes more than 50 miles from a sorting facility get fewer trips and might not be sorted on the same day that the mailbox contents are retrieved, there will be a one-day delay on most days, a two-day delay on Saturdays or the day before a holiday (because they won't postmark on Sundays or holidays), or a three-day delay on Saturdays before a Monday holiday (because the Sunday plus holiday represents two consecutive days they don't sort mail).

One easy thing to implement, for anyone whose procedures depend on the postmark, is to just move the latest allowable postmark to be the next business day after the deadline, between 1-3 days. Another could be to inform customers that if they live in one of the affected zip codes, they need to affirmatively ask for a same-day postmark at the retail location where they're dropping off the mail. That might require some additional driving to the actual post office instead of an unattended drop box, but in most places the latest pickup in a blue box is actually earlier than the time the nearest post office closes anyway.

And I will say that I've gamed the system before, where I bought a barcode based stamp just before midnight from an automated postal machine, because those aren't postmarked at all, and most recipients incorrectly consider the purchase date on the stamp to be equivalent of the postmark date, so that I had a few extra hours to finalize an application past midnight that night.

Just like with Social Security numbers, where Social Security Administration had to tell everyone "wait this number wasn't intended to be used in that way by others," this seems to be the USPS pushing back against certain assumptions on postmark dates.

What's the fee for late forms? Or, more likely, late payments? Another way to squeeze a few bucks from poor people?