title: DateTimeOffset?.ToString(string format)
body: <p>I have a <code>DateTimeOffset?</code> object which I want to format according to some datetime format. </p>
  
  <p>I found that type <code>DateTimeOffset</code> has <code>ToString(String,IFormatProvider)</code> method:</p>
  
  <p><a href="http://msdn.microsoft.com/de-de/library/bb351892%28v=vs.110%29.aspx" rel="nofollow">http://msdn.microsoft.com/de-de/library/bb351892(v=vs.110).aspx</a></p>
  
  <p>But neither does this exist for an object of type <code>DateTimeOffset?</code>, nor can I find specifics on this type, or how to convert it.</p>
  
  <p>What is <code>DateTimeoffset?</code>, and how do I convert it to a string using custom format?</p>