That's not fair!!
Artist's commentary
Hang in There, Kogasa-san 3406
That would be after the last mail pickup was done, though. I think it was right around 1:00 a.m.?
When you change out the date, you have to record it properly, so you can't change it back. Well, not that you couldn't, but there'd probably be penalties if you did.
Also, you could pass through mail addressed to you right on the spot, so if you were lucky, you could receive it as fast as you would get Express Mail! :-) I don't know what it's like these days, though!