permalink changes when post date is changed.

One of my members made a blog post private while working on it, and then subsequently made it public in lieu of publishing it. I edited the date of the post to reflect the publication date rather than the date he started writing it, and the permalink to the post changed because the url contains the month number. There was no warning or indication that this problem would arise when I changed the date, and I didn’t find out until after I received complaints from the author about dead links. Is this a bug or have I made some mistake?