This is just a guess, but . . the PC's handle all the mail on the ship, and they are the ones who do the postmarking. So, lets say for some reason you were stupid and went to mail it on July 16th this year, well that just wouldn't work right? So, I'm guessing a thrifty person may be able to speak with a thrifty PC who happens to have a postmark that he forgot to move from July 15th when he stamps the package.
Just a guess, and I am in no way saying that anyone should be so dumb as to screw this one up, but to each his own I suppose.