MDX Months Since a End Date

Hi, I'm very new to MDX, and struggling to get the MDX to calculate months since an End Date, Im not even sure whether to use the End Date stored in the measure or the End Date stored in the dimension. The goal is to find the months of inactivity. 

1. we have an 'End Date' in the cube for each customer (stored both as fact and also as a dimension - don't know which one to use)

2. need to work out dynamically how many months from that stored End Date to the last month that we have sliced from the cube, i.e. the last month in the WHERE clause slicer.

I have spent two days trying to figure this out and really need help!

thanks in advance