IIRC, the military postal system postmarks don't have a date stamp, that being the issue in the Florida vote in 2000.
They have one, it's just that they didn't always use them especially on ships where all the mail is placed in a bundle before being shipped to the USPS. Indeed, "MPS" mail franked with ZIP codes of now-defunct military bases are somewhat valuable to stamp collectors. I think they are now supposed to date stamp all election ballots.
Besides, for the date stamp to come into play, the ballot would have had to have been received after midnight of the 6th at the earliest for it to be rejected as arriving "too late." At the time of the initial posting, it was not yet midnight anywhere in the United States at any polling place, so how could it be "rejected as too late"?