title: Recording gaps between two times
body: <p>Below given, is a set of data from one of the tables I have, Is there any way to track the empty time? (e.g  (11:00-12:00) and (18:45-19:15) as gap for that date).</p>
  
  <pre><code>       Date           Activity  From     To    Duration
  2013-09-09 00:00:00.000 Work    08:00   11:00   3.00
  2013-09-09 00:00:00.000 Travel  12:00   13:00   1.00
  2013-09-09 00:00:00.000 Work    13:00   15:00   2.00
  2013-09-09 00:00:00.000 Work    15:00   18:00   3.00
  2013-09-09 00:00:00.000 Travel  18:00   18:45   0.75
  2013-09-09 00:00:00.000 Work    19:15   22:15   3.00
  </code></pre>
  
  <p>I'm expecting something like below</p>
  
  <pre><code>       Date           Activity  From     To    Duration
  2013-09-09 00:00:00.000 Work    08:00   11:00   3.00
  2013-09-09 00:00:00.000 Gap     11:00   12:00   1.00
  2013-09-09 00:00:00.000 Travel  12:00   13:00   1.00
  2013-09-09 00:00:00.000 Work    13:00   15:00   2.00
  2013-09-09 00:00:00.000 Work    15:00   18:00   3.00
  2013-09-09 00:00:00.000 Travel  18:00   18:45   0.75
  2013-09-09 00:00:00.000 Gap     18:45   19:15   0.50
  2013-09-09 00:00:00.000 Work    19:15   22:15   3.00
  </code></pre>
  
  <p>Can this be done without looping? If so,? how can it be done?</p>