There are more than one way to do it, you can make some changes in data model to simplify your set analysis expressions as discussed in detail Year-over-Year Comparisons
Another approach you can take it do comparison in set analysis of the expression. For that you have to give range of months you want to include while calculating, in your case it can be last 12 months or years as 2013/2014/2015 so on.
You can find some more details in Range of month in a bar chart . It is for bar charts but your expression will remain same for straight table as well.