Calculating Months between two dates Pls Help I'm going mad!

I've got a sheet where I'm tracking when to update certain forms etc.
The formula I am using is
=IF(H6="","",(YEAR(AB7)-YEAR(H7))*12+MONTH(AB7)-MONTH(H7))

H6= Date due for Revision
AB6= Todays Date (this updates each time the sheet is opened)

H6= 2/11/2009
AB6= 28/7/2011 (Todays date)

The document is overdue for revision by years, however the result of my formula is 29 instead of -29

If I write the formula as
=IF(H6="","",(YEAR(H7)-YEAR(AB7))*12+MONTH(H7)-MONTH(AB7))
Then I get the correct result.

The formula works great, however the Date due for revision is not always current or in the future, sometimes it is in the past which is causing the formula not to pick up the negative figures when neccesary. These dates will always be changing so I can't just simply swap the order of the cells that are being deducted from each other as I please. I need something that works correctly no matter the order of the dates.

I hope this makes sense