title: Requiring timezone information when deserialising DateTimeOffset
body: <p>I am trying to validate data I get from an API endpoint. The user should be able to send in a <code>DateTimeOffset</code>. Currently I am using my own implementation of <code>JsonConverter</code> to validate the correct format of the <code>DateTimeOffset</code>. I have tried multiple date time formats but none of them quite work as I am wanting.</p>
  
  <pre><code>"yyyy-MM-ddTHH:mm:ss.fffK"
  </code></pre>
  
  <p>This works for: </p>
  
  <pre><code>2010-01-01T01:01:01.000
  2010-01-01T01:01:01.000Z
  2010-01-01T01:01:01.000+01:00
  </code></pre>
  
  <p>The problem is, it is not enforcing a time offset or Z to be specified as seen in the top example, apart from that it works great.</p>
  
  <pre><code>"yyyy-MM-ddTHH:mm:ss.fffzzz"
  </code></pre>
  
  <p>This works for only: </p>
  
  <pre><code>2010-01-01T01:01:01.000+01:00
  </code></pre>
  
  <p>It doesn't accept Z as a time offset. Is there a <code>DateTimeOffset</code> format which I could specify to only accept the following:</p>
  
  <pre><code>2010-01-01T01:01:01.000Z
  2010-01-01T01:01:01.000+01:00
  </code></pre>