title: calculate time difference in minute between each record in group
body: <p>I have following table:</p>
  
  <pre><code>Event   Startdatetime          Enddatetime            Value
  1       '23/09/2016 12:15:20'  '23/09/2016 12:34:30'  50
  1       '23/09/2016 14:10:40'  '23/09/2016 14:30:25'  40
  2       '25/10/2016 10:20:45'  '25/10/2016 10:45:55'  80
  2       '25/10/2016 11:27:35'  '25/10/2016 11:48:55'  30
  </code></pre>
  
  <p>Each record has startdatetime and enddatetime and I'd like to calculate time difference in minute between each record in group (by event).</p>