title: Powerpivot - Determine how much a user-input date changes over time
body: <p>I have a table with a series of monthly measurements for several cases. One of the measures per case is the date on which an analyst predicts that case will close (the Forecast Date). </p>
  
  <p>In Month 1, an analyst might think a case is going to close on June 30, 2015. But then in Month 2, that analyst might revise the estimate and say that case will close on August 15. </p>
  
  <p>What I want to do is create a calculated column that tells me the difference between last month's Forecast Date and this month's Forecast Date.</p>
  
  <p>Assuming Month 1 is Jan 2015, what I want is the diff column:</p>
  
  <pre><code>CaseNum | MeasureMonth | ForecastDate | DiffFromPrevMeasure 
  case1   | 2015/01/31   | 2015/06/30   |-- (there is no previous date)
  case1   | 2015/02/28   | 2015/08/15   | 46
  case1   | 2015/03/31   | 2015/08/01   | -14
  </code></pre>
  
  <p>And so on. There are obviously many cases in the table. </p>
  
  <p>I need to do things like examine the distribution of changes, show what % of changes are +/- before and after a certain date, and stuff like that. So I can't do it via a measure.</p>
  
  <p>I am thinking I need to do something with CALCULATE / MAX / EARLIER minus current row's date, but I just can't make it work.</p>
  
  <p>I'm using Excel 2010, not 2013, if that's important to the solution.</p>