JavaScript Countdown

countdown.js

function getTimeRemaining(endtime) {
  var t = Date.parse(endtime) - Date.parse(new Date());
  var seconds = Math.floor((t / 1000) % 60);
  var minutes = Math.floor((t / 1000 / 60) % 60);
  var hours = Math.floor((t / (1000 * 60 * 60)) % 24);
  var days = Math.floor(t / (1000 * 60 * 60 * 24));
  return {
    'total': t,
    'days': days,
    'hours': hours,
    'minutes': minutes,
    'seconds': seconds
  };
}

function initializeClock(id, endtime) {
  var clock = document.getElementById(id);
  var daysSpan = clock.querySelector('.days');
  var hoursSpan = clock.querySelector('.hours');
  var minutesSpan = clock.querySelector('.minutes');
  var secondsSpan = clock.querySelector('.seconds');

  function updateClock() {
    var t = getTimeRemaining(endtime);

    daysSpan.innerHTML = t.days;
    hoursSpan.innerHTML = ('0' + t.hours).slice(-2);
    minutesSpan.innerHTML = ('0' + t.minutes).slice(-2);
    secondsSpan.innerHTML = ('0' + t.seconds).slice(-2);

    if (t.total <= 0) {
      clearInterval(timeinterval);
    }
  }

  updateClock();
  var timeinterval = setInterval(updateClock, 1000);
}

var deadline = new Date(Date.parse(new Date()) + 31 * 20 * 50 * 60 * 1000);
initializeClock('clockdiv', deadline);

index.html

<div id="clockdiv">
	<div class="singleTime">
		<div>
			<span class="days"></span>
		</div>
		<div class="smalltext">Days</div>
	</div>
	<div class="singleTime">
		<div>
			<span class="hours"></span>
		</div>
		<div class="smalltext">Hours</div>
	</div>
	<div class="singleTime">
		<div>
			<span class="minutes"></span>
		</div>
		<div class="smalltext">Minutes</div>
	</div>
	<div class="singleTime">
		<div>
			<span class="seconds"></span>
		</div>
		<div class="smalltext">Seconds</div>
	</div>
</div>

style.css

#clockdiv{
	font-family: sans-serif;
	font-weight: 100;
	font-size: 30px;
	display: inline-block;
}
.singleTime{
	display: inline-block;
    width: 100px;
    overflow: hidden;
    margin: 0 10px;
}
.singleTime div{
	background: #f39c12;
    color: #fff;
    padding: 20px;
    border-radius: 3px;
    font-size: 32px;
}
.singleTime div span{
	border: 1px solid #fff;
    padding: 8px 15px 10px 9px;
    border-radius: 3px;
}
.singleTime .smalltext{
	background: transparent;
	color: #3c4a62;
	font-size: 20px;
}

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How to change or add time in js?