title: Format Column Header on pivot table as date in ag-grid
body: <p>I am using a valueFormatter for my date columns in ag-grid (react) that looks like this:</p>
  
  <pre><code>columnTypes: {
                  "dateColumn": {
                      filter: 'agDateColumnFilter',
                      enableRowGroup: true,
                      valueFormatter: Blotter.formatDate,
                      enablePivot: true,
                      enableValue: true                 
                  }
  }
  </code></pre>
  
  <p>where Blotter.formatDate is a simple formatter that outputs the date in the user's preferred format.  However, a common requirement in my application is to build pivot tables using a date series so the date column ends up in the "Column Labels" section of the pivot definitions. When I do this, the date column header shows the full, unformatted date and is also not sorted in any particular order. How can I make it so that the column label formats the dates in a reasonable way and have them sorted incrementing from left to right?</p>
  
  <p>Here's what I'm seeing:
  <a href="https://i.stack.imgur.com/CaPbN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CaPbN.png" alt="enter image description here"></a></p>
  
  <p>The dates across the top are from the "Position Date" column, which renders like this when pivot is not turned on:</p>
  
  <p><a href="https://i.stack.imgur.com/KdvQY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KdvQY.png" alt="enter image description here"></a></p>
  
  <p>Thankyou,
  Troy</p>