<p class="net" id="date"></p>

.net {
  text-align: center;
  font-size: 20px;

}

var countDownDate = new Date("May 11, 2040 09:40:00").getTime();
var x = setInterval(function() {
var now = new Date().getTime();
var distance = countDownDate - now;
var days = Math.floor(distance / (1000 * 60 * 60 * 24));
var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
var seconds = Math.floor((distance % (1000 * 60)) / 1000);
document.getElementById("date").innerHTML = days + "d " + hours + "h "
+ minutes + "m " + seconds + "s ";
if (distance < 0) {
clearInterval(x);
document.getElementById("date").innerHTML = "It's over";
  }
}, 1000);

<iframe src="https://www.netbax.net/tab/javascript/countdown-timer.html" style="width: 550px; height:550px;" frameborder="0"></iframe>