Under the new rule, USPS no longer stamps mail when they receive it. Instead, the “official” postmark date is now whenever your envelope first hits an automated sorting machine at a regional processing center.  Your mail might sit in a truck for a day or two before it gets there. Drop something in your local mailbox on Tuesday, get a Thursday postmark. The postal service admits the date is no longer a “perfectly reliable indicator” of when they actually took possession.