title: Moment js diff with different timezone
body: <p>I want to calculate the difference of time in hours between 2 different time zones using the moment.js library.</p>
  
  <p>I tried using <code>diff</code> function provided by the library but it is not providing correct answers, I tried:</p>
  
  <pre><code>moment('2016-12-18 6:00:00', 'YYYY-MM-DD HH:mm:ss')
      .diff(moment('2016-12-18 6:00:00', 'YYYY-MM-DD HH:mm:ss').utcOffset('+09:30'), 'hours')
  </code></pre>
  
  <p>I am in <code>UTC+5:30</code> and the <code>moment(string, string)</code> constructor returns the time in my current timezone. And I was expecting the answer to be <code>4</code> instead got <code>0</code>. Is there any other function that moment library provides to get difference amongst timezones ?</p>