title: moment.js diff between two timezone formatted dates
body: <p>I have to calculate a time difference between a set time and the current server time.</p>
  
  <p>I'm using the momentjs and the moment timezone library.</p>
  
  <p>Here's what I have: </p>
  
  <pre><code>var serverTime = moment.tz(new Date(), "Europe/Berlin").format();
  var bar = moment.tz("2016-05-14 00:00:00", "Europe/Berlin").format();
  var baz = bar.diff(serverTime);
  
  console.log('server time is: ' + serverTime);
  console.log('time diff is: ' + baz);
  </code></pre>
  
  <p>This gives me an error:</p>
  
  <pre><code>TypeError: bar.diff is not a function
  </code></pre>
  
  <p>Is it possible to diff between two timezone formatted dates? If not, what's the optimal way to conduct such calculation using the momentjs library?</p>