Need to calculate individual work experience in years and months within excel.

Hello All,
My name is will and I am losing my mind trying to figure this out. My company would like to keep track of every individuals years and months of experience dating back to when they first started, and it only wants to calculate relevant experience. On top of that, they want it to be self updating so that it is always current. Here is my issue, I have no idea what I am doing. While researching I found that if I use the DATEDIF formula it will calculate the period between two dates. This is good, however, look at this example:
John Doe work at one company from 12/2003 until 08/2006 and his experience is valid. He also worked from 09/2006 until 11/2008 where his experience does not count towards his overall "relevant" experince. He then continued to work from 12/2008 until present where his experience is relevent.
So, as you can see I need excel to calculate the years and months from 12/2003-Present, while omitting the dates where his experience is not relevent (09/2006-11/2008). I tried a work around which was to calculate the first employment history using DATEDIF and then do the third, however, when I try to add the two seperate results I get an error. Any help with this would be much appreciated. Attached is the example in excel.

Thanks
Will
Example Work Experience.xlsx