Here in Hawai’i, the mail slots have the pickup times printed on them. So if you drop something in the mail at 1pm on a Monday, and the box says “mail picked up at 2pm” you know you’ll get a post mark the same day. If it says 2pm and you drop it in the slot at 4pm, you know the postmark will be Tuesday.
(Or maybe it’s been that way everywhere in the US for years, and I’m being Captain Obvious. It’s happened before.)
All I care about this, not being in the US, is that they use the ISO 8601 standard for dates.
They do that here too.
However, for years now, most and now maybe all mail goes to the nearest large city, nearly two hours away, for processing. AIUI, this latest change means it won’t be postmarked at the local office, but not until it gets to the city. But I think that pickup time is for when the local office takes it out of the box to send it to the city; so changing the postmark location means that time won’t apply any longer.