Month on Month - % difference

Perhaps i didn't explain well.  [Post Count] wasn't supposed to be a field but an example of a measure.  What you need to do is a create a measure that performs your KPI, knowing that it doesn't handle the correct time constraint.  The calculate statement will run your measure and force in your applicable dates.

You will notice in my example I created the measure Post Count, which does a count of the number of rows in the data set.  The expectation here, for my example, was that each post would be a single row in the data.  Thus if I was to count my pretend data, i could get a count of all the posts in a given time period.