title: Pivot table cannot find difference between January current year to December of previous year. Excel 2010
body: <p>I am weeks into making my first ever pivot tables, cant believe the power of them but have hit a snag I cannot overcome.</p>
  
  <p>I have a worksheet which has a column with raw data (manually imputed)</p>
  
  <p>Then I have another 2 columns that calculate the difference from the previous month and the % difference. This works fine.</p>
  
  <p><img src="https://i.stack.imgur.com/uOsvd.jpg" alt="Sample of the table (note changed some numbers for data security so the calculations don&#39;t add up)"></p>
  
  <p>As you can see January 2015 is blank as the difference field is looking for the previous month and January is the first month.</p>
  
  <p><img src="https://i.stack.imgur.com/KU8jH.jpg" alt="enter image description here"></p>
  
  <p>how can I get it to understand to look at December 2014</p>