title: get difference of four date column
body: <p>Code for 2 date columns difference query is fine.
  but when i want to join their result, i am getting error</p>
  
  <pre><code>CONVERT(VARCHAR(12), DATEADD(MS, DATEDIFF(MS,Stat_EndTime, Stat_StartTime), 0), 108) AS [VisitTime],
  CONVERT(VARCHAR(12), DATEADD(MS, DATEDIFF(MS,Load_EndTime, Load_StartTime), 0), 108) AS [LoadTime] 
  </code></pre>
  
  <p>When i did a join as below for the complete difference time, i am getting error </p>
  
  <pre><code>CONVERT(VARCHAR(12), DATEADD(MS, DATEDIFF(MS,Stationary_EndTime, Stationary_StartTime), 0), 108) - CONVERT(VARCHAR(12), DATEADD(MS, DATEDIFF(MS,LoadingEndTime, LoadingStartTime), 0), 108) AS [Actual Contact Time],
  </code></pre>
  
  <blockquote>
    <p>The data types varchar and varchar are incompatible in the subtract operator.</p>
  </blockquote>