DATEIF: Count months between dates when...

Hi All,

Thanks in advance for all the help. Here is my situation.

In my spreadsheet I have a start and and end date. I need to count the months included in that span. Here's the twist - I have some that span multiple months, multiple years, and some that span only a few dates in one month. I need even one day to count as a month.

i.e.
1/1/13 - 12/31/14 = 24 months
1/1/13 - 2/1/13 = 2 months
1/1/13 - 1/15/13 = 1 month (NOT ZERO)

How do I do this???????