title: Difference of dates between two dates starting everytime at first row date
body: <p>I have table like this </p>
  
  <p>I got difference from consecutive dates but I want to calculate difference of dates starting every time at 1. like (1-2 and 1-3 again 1-4)      </p>
  
  <pre><code>ChildID   ID      Date
  1         4    2005-12-07 00:00:00.000
  1         3    2005-11-07 00:00:00.000
  1         2    2005-10-09 00:00:00.000
  1         1    2005-04-09 00:00:00.000
  </code></pre>
  
  <p>Expected </p>
  
  <pre><code>ChildID   ID      Date                         Difference
  1         4    2005-12-07 00:00:00.000            243 days (1-4)
  1         3    2005-11-07 00:00:00.000            207 days (1-3)
  1         2    2005-10-09 00:00:00.000            183 days (1-2)
  1         1    2005-04-09 00:00:00.000            NULL 
  </code></pre>