title: ToUnixTimeSeconds method missing after migration to VS 2017
body: <pre><code>var item = new DateTimeOffset(DateTime.UtcNow).ToUnixTimeSeconds();
  </code></pre>
  
  <p>The code causes the compilation error <strong>"Does not contain a definition for ToUnixTimeSeconds..."</strong>.
  It works well in VS 2015, also I have <code>using System;</code> namespace and references to both mscorlib and System (4.0.0.0 version). Many other members of DateTimeOffset exist.</p>