mm is not MM

Even though when you call a ToString() on a DateTime the day and year format characters are dd and yyyy, respectively, that does not mean that the month part is mm.  mm is already used to format for minute.  I spent a while today trying to figure out why I was getting a number that was beyond the range of 1-12 for the mm portion of my date when using ToString(mmddyyyy).  You have to use MM for your month portion.