Compare to previous year within section

I want to create a simple table calculation that compares values directly next to one another without crossing into the next month.  For example, in the image below, I'd like to calculate % change from 

Feb 2009 to 2010, from Feb 2010 to 2011, and so on, but I do not want to calculate Jan 2012 to Feb 2009 just because they are next to one another.  Is there a way to add an "IF" clause to prevent that calculation?

I've included the .twbx file I used to generate the screenshot.