title: Format Diffrence time between two DateTime objects
body: <p>I've got a two <code>DateTime</code> objects - <code>StartDate</code> and <code>EndDate</code>
  I would like format difference between two dates like <code>HH:mm:ss</code>.</p>
  
  <pre><code>EndTime.Subtract(StartDate).Hours + ":" +
  EndTime.Subtract(StartDate).Minutes + ":" +
  EndTime.Subtract(StartDate).Seconds
  </code></pre>
  
  <p>This works fine but looks ugly, I tried like this:</p>
  
  <pre><code>EndTime.Subtract(StartDate).ToString("HH:mm:ss")
  </code></pre>
  
  <p>but this threw Exception:</p>
  
  <blockquote>
    <p>Input string was not in a correct format</p>
  </blockquote>
  
  <p>What I'm doing wrong?
  I Would like have format like this - <code>01:55:23</code></p>