On April 15 of this year, I went to my local APC at 10:30 PM, long after the actual post office had closed. My intent was to buy ten first-class stamps and mail them in succession, seeing how old the stamps would have to be before the letters would start being returned, as well as whether or not they would be cancelled with an additional, dated postmark.
So I arranged with friends a thousand miles away (in Seattle) to receive the letters, and as a control subject, sent one letter that night of April 15. The next letter was sent the next day. …And so on, at increasing intervals of time, through April 29, a full two weeks after the date of the stamp. I expected that letters sent in the first week or so would arrive, and then they’d start coming back.
I was wrong. They all made it.
The interesting part was that, as predicted, not all of the stamps arrived with cancellations. Of the ten sent to Seattle, only six arrived there cancelled — meaning that four envelopes (40%) arrived indicating only the April 15 date and no other postmark.
Wonder how long 'till Bruce Schneier takes note.