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<div class="container">
  <h1 id="head">Countdown to my birthday:</h1>
  <ul>
    <li><span id="days"></span>days</li>
    <li><span id="hours"></span>Hours</li>
    <li><span id="minutes"></span>Minutes</li>
    <li><span id="seconds"></span>Seconds</li>
  </ul>
</div>

const second = 1000,
      minute = second * 60,
      hour = minute * 60,
      day = hour * 24;

let countDown = new Date('Sep 30, 2020 00:00:00').getTime(),
    x = setInterval(function() {

      let now = new Date().getTime(),
          distance = countDown - now;

      document.getElementById('days').innerText = Math.floor(distance / (day)),
        document.getElementById('hours').innerText = Math.floor((distance % (day)) / (hour)),
        document.getElementById('minutes').innerText = Math.floor((distance % (hour)) / (minute)),
        document.getElementById('seconds').innerText = Math.floor((distance % (minute)) / second);
      
      //do something later when date is reached
      //if (distance < 0) {
      //  clearInterval(x);
      //  'IT'S MY BIRTHDAY!;
      //}

    }, second)

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  background-color: #ffd54f;
}

.container {
  color: #333;
  text-align: center;
}

h1 {
  font-weight: normal;
}

li {
  display: inline-block;
  font-size: 1.5em;
  list-style-type: none;
  padding: 1em;
  text-transform: uppercase;
}

li span {
  display: block;
  font-size: 4.5rem;
}














/* general styling */
html, body {
  height: 100%;
  margin: 0;
}

body {
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center;
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  font-family: -apple-system, 
    BlinkMacSystemFont, 
    "Segoe UI", 
    Roboto, 
    Oxygen-Sans, 
    Ubuntu, 
    Cantarell, 
    "Helvetica Neue", 
    sans-serif;
}

.container {
  margin: 0 auto;
}