Well, I don't think there's any well-defined standard for what that would
mean. The length of a month varies, after all. How many months are between Jan 31 and March 1? How many between March 1 and March 31? There are several ways you could answer those questions, and it's not clear that one answer is "right".
If you want a simple appoximation, it's easy:
Otherwise you probably need to define precisely what a month
is. Good luck.