How do I show change in a measure

[Minty]

I have a simple matrix giving the value of a measure for this month and for last month.  I want to show visually whether this has gone up, down or stayed the same.  How can I do this?

I tried to create another measure doing one minus the last based on various of my period/date fields but it wouldn't recognise any of them as dates.

[ahmedoye]

Difference = [Measure] - CALCULATE([Measure], PREVIOUSMONTH(MyTable[Date].[Date]))