title: creating a calculated field in excel pivot table based on an item in a column
body: <p>Normally calculated pivot table fields in excel (2010) you click: 
  design tab > options and sets > calculated field</p>
  
  <p>and if you had three columns: name, sales, cost</p>
  
  <pre><code>name | sales | cost
  josh    10      2    
  </code></pre>
  
  <p>your calculated field might be: profit which would be: = sales - cost.</p>
  
  <p>However is it possible to make a calculated field which calculates the difference between two items in the one "Week Ending" column?</p>
  
  <p>For example "Week ending" contains a dates which end on a friday 2015-05-01,2015-05-08,2015-05-15 etc and they are in the column labels section of the pivot table.</p>
  
  <p>The "week ending" dates are all in the one column, is it still possible to calculate the difference between two of them with a calculated field?</p>
  
  <pre><code>name | End Fri 2015-05-01 | End Fri 2015-05-08 | calculated (Delta)
  josh    8                              10           10-8 = 2
  </code></pre>