Excel Formula/Function to find total number of non-overlapping months

Hi Guys, good day to you all. I have 10 projects, each with a start and an end date. I need to find the number of months in between each project's start date and end date without counting overlapping months. For example;

Project one, I count three months from Jan 2015 - Mar 2015
If project two runs from Jan 2015 - Jun 2015, I wont be counting 6 months but 3 months as I wont count the overlapping months.

I have attached an excel which has a sample data.

Thanks.