title: TimeSpan ToString "[d.]hh:mm"
body: <p>I trying to format a TimeSpan to string.
  Then I get expiration from MSDN to generate my customized string format.
  But it don't words. It returns "FormatException".</p>
  
  <p>Why? I don't understand...</p>
  
  <pre><code>var ts = new TimeSpan(0, 3, 25, 0);
  var myString = ts.ToString("[d'.']hh':'mm");
  </code></pre>