(function() {
  var deadline = '2021/12/30 14:15';

  function pad(num, size) {
    var s = "0" + num;
    return s.substr(s.length - size);
  }

  function getTimeRemaining(endtime) {
    var t = Date.parse(endtime) - Date.parse(new Date()),
      seconds = Math.floor((t / 1000) % 60),
      minutes = Math.floor((t / 1000 / 60) % 60),
      hours = Math.floor((t / (1000 * 60 * 60)) % 24),
      days = Math.floor(t / (1000 * 60 * 60 * 24));

    return {
      'total': t,
      'days': days,
      'hours': hours,
      'minutes': minutes,
      'seconds': seconds
    };
  }

  function clock(id, endtime) {
    let hours = document.getElementById(id + '-hours')
    let minutes = document.getElementById(id + '-minutes');

    var timeinterval = setInterval(function() {
      var time = getTimeRemaining(endtime);

      if (time.total <= 0) {
        deadline = "een datum"
        clearInterval(timeinterval);
      } else {
        hours.innerHTML = pad(time.hours, 2);
        minutes.innerHTML = pad(time.minutes, 2);
      }
    }, 1000);
  }

  clock('js-clock-header', deadline);

  function clocks(id, endtime) {
    let hours = document.getElementById(id + '-hours')
    let minutes = document.getElementById(id + '-minutes');

    var timeinterval = setInterval(function() {
      var time = getTimeRemaining(endtime);

      if (time.total <= 0) {
        clearInterval(timeinterval);
      } else {
        hours.innerHTML = pad(time.hours, 2);
        minutes.innerHTML = pad(time.minutes, 2);
      }
    }, 1000);
  }

  clocks('js-clock', deadline);
})();

<span id="js-clock-header-hours"></span>:<span id="js-clock-header-minutes"></span><br />
<span id="js-clock-hours"></span>:<span id="js-clock-minutes"></span>