title: moment.tz is giving incorrect result
body: <p>I was trying to convert dates according to selected timezone. I was surprised to see same result for dates with 5 mins of time difference.
  For ex,</p>
  
  <pre><code>var x = "2017-07-10T18:30:00.000Z"
  var y = "2017-07-10T18:35:00.000Z"
  var z = "2017-07-10T18:45:00.000Z"
  </code></pre>
  
  <p>and tried converting them to using moment.tz:</p>
  
  <pre><code>moment.tz(x, 'America/New_York').format('DD/MM/YYYY HH:MM:SS');
  moment.tz(y, 'America/New_York').format('DD/MM/YYYY HH:MM:SS')
  moment.tz(z, 'America/New_York').format('DD/MM/YYYY HH:MM:SS')
  </code></pre>
  
  <p>To my surprise, result was same for all 3 being <code>"10/07/2017 14:07:00"</code>.
  What's going wrong? Any help will be appreciated.  </p>