Setup

Setup is as simple as doing the following:

//HTML

<p id="timer"></p>

Create php file to return time server time:

<?php

// get_servertime.php

date_default_timezone_set('Europe/London');

header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Fri, 1 Jan 2010 00:00:00 GMT"); // Date in the past
$now = time() * 1000;

$end = strtotime( $_GET['date'] ) * 1000;

# We only need the difference in dates
echo $end - $now;
?>

// JavaScript

var dateTo = "January 25, 2016 12:00:00",
	get_server_time = function() {
	    var time = null;
		$.ajax({
			url: 'get_serverdate.php',
			data : {
				date: dateTo
			},
			async: false,
			dataType: 'json',
			success: function( date, status, xhr ) {
				time = date;
			},
			error: function(xhr, status, err) {
				time = new Date();
				time = time.getTime();
			}
		});
		return time;
	},
	time = get_server_time();

$("#time").countdown({
	date: dateTo,
	serverDiff: time,
	yearsAndMonths: true
});