title: C# DateTime calculation and ToString
body: <p>Im doing simple calculation with time, to see how long the process has been running. </p>
  
  <pre><code>(DateTime.Now - StrtTime).ToString("hh:mm:ss")
  </code></pre>
  
  <p>Where StrtTime is:<code>DateTime StrtTime = DateTime.Now;</code>
  . But im getting an exception:</p>
  
  <pre><code>Input string was not in a correct format.
  </code></pre>
  
  <p>Whats the proper way to do this?</p>