This is the nature of MTD. The question is - how do you want it to work?
Today is February 28, 2012. Previous MTD should include Jan 28. Is it correct? Why?
Today is February 29, 2012. Previous MTD should include Jan 29. Or should it be Jan 31? Why or why not? How can you tell which one is correct? They both can be correct, depending on what you're looking for.
Maybe you need a switch showing if you want the full or partial previous month, and use a conditional calculation.