moment.js + moment-timezone.js

gistfile1.txt

<h3>Converting current time</h3>
<div>
  UTC: <span class="align" id="utc"></span>
</div>

<div>
  London: <span class="align" id="london"></span>
</div>

<div>
  Paris: <span class="align" id="paris"></span>
</div>

<div>
  New York: <span class="align" id="new-york"></span>
</div>

<h3>Converting an instant</h3>

<div>
  UTC: <span id='some-utc-instant'></span>
  <span id='friendly'></span>
</div>

<div>
  New York: <span id='some-local-instant'></span>
</div>

setInterval(renderTime, 1000);

function renderTime() {
	// Troubleshooting: https://maggiepint.com/2016/05/14/moment-js-shows-the-wrong-date/
	
	var utc = moment().utc();
	
	$('#utc').text(utc);
	$('#london').text(moment.tz(utc, 'Europe/London'));
	$('#paris').text(moment.tz(utc, 'Europe/Paris'));
	$('#new-york').text(moment.tz(utc, 'America/New_York'));
	
	
	var someDate = moment('2017-06-30T23:30:00+01:00');
	$('#some-utc-instant').text(someDate);
	$('#some-local-instant').text(moment.tz(someDate, 'America/New_York'));
	$('#friendly').text('This was ' + someDate.from(utc));
}