That strengthens the case for having the user type in the value rather than using a drop-down menu. That way you can do the heavy lifting of normalizing the format rather than forcing the user to do it in their head.

This might seem really petty, but I'm always a little annoyed when I have to mentally convert the number on my credit card into a month. It always takes a moment to remember that 04 is March, or whatever. It's much easier just to type the value.