countdown.js

var comingdate = new Date("Jan 1, 2019 00:00:00");

var d = document.getElementById('d');
var h = document.getElementById('h');
var m = document.getElementById('m');
var s = document.getElementById('s');

var x = setInterval(function(){
    var now = new Date();
    var des = comingdate.getTime() - now.getTime();
    
        var days = Math.floor(des / (1000 * 60 * 60 * 24));
        var hours = Math.floor(des % (1000 * 60 * 60 * 24) / (1000 * 60 * 60));
        var mins = Math.floor(des % (1000 * 60 * 60) / (1000 * 60));
        var secs = Math.floor(des % (1000 * 60) / 1000);
    if(des<=0) {
        days=0;
        hours=0;
        mins=0;
        secs=0;
    }
   
    d.innerHTML = getTrueNumber(days);
    h.innerHTML = getTrueNumber(hours);
    m.innerHTML = getTrueNumber(mins);
    s.innerHTML = getTrueNumber(secs);

    if(des <= 0) clearInterval(x);

},1000);

function getTrueNumber(x) {
    if (x<10) return '0'+x;
    else return x;
}