title: Why does TimeSpan.ParseExact not work
body: <p>This is a bit wierd. Parsing a text field with a valid timespan fails if I try to be precise!</p>
  
  <pre><code>const string tmp = "17:23:24";
  //works
  var t1 = TimeSpan.Parse(tmp);
  //fails
  var t2 = TimeSpan.ParseExact(tmp, "hh:mm:ss", System.Globalization.CultureInfo.InvariantCulture);
  </code></pre>
  
  <p>The second parse fails with an exception "Input string was not in a correct format." from DateTime.</p>