File: public/assets/js/countdown-timer.js

// Count down milliseconds = server_end - server_now = client_end - client_now
var server_end = <?php echo $exp_date; ?> * 1000;
var server_now = <?php echo time(); ?> * 1000;
var client_now = new Date().getTime();
var end = server_end - server_now + client_now; // this is the real end time

var _second = 1000;
var _minute = _second * 60;
var _hour = _minute * 60;
var _day = _hour *24
var timer;

function showRemaining()
{
    var now = new Date();
    var distance = end - now;
    if (distance < 0 ) {
       clearInterval( timer );
       document.getElementById('countdown').innerHTML = 'EXPIRED!';

       return;
    }
    var days = Math.floor(distance / _day);
    var hours = Math.floor( (distance % _day ) / _hour );
    var minutes = Math.floor( (distance % _hour) / _minute );
    var seconds = Math.floor( (distance % _minute) / _second );

    var countdown = document.getElementById('countdown');
    countdown.innerHTML = '';
    if (days) {
        countdown.innerHTML += '<div class="day">Days: <span class="time">' + days + '</span>&nbsp; </div> ';
    }
    countdown.innerHTML += '<div class="hours">Hours: <span class="time">' + hours+ '</span>&nbsp;</div> ';
    countdown.innerHTML += '<div class="minutes">Minutes: <span class="time">' + minutes+ '</span>&nbsp;</div> ';
    countdown.innerHTML += '<div class="seconds">Seconds: <span class="time">' + seconds+ '</span>&nbsp; </div>';
}

timer = setInterval(showRemaining, 1000);