app.js

const endDate = new Date(2022, 0, 5, 19, 0, 0).getTime();

// countdown
let timer = setInterval(function() {

  // get today's date
  const today = new Date().getTime();

  // get the difference
  let diff = endDate - today;

  let days, hours, minutes, seconds;
  
  if (diff > 0) {
    // math
    days = Math.floor(diff / (1000 * 60 * 60 * 24));
    hours = Math.floor((diff % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    minutes = Math.floor((diff % (1000 * 60 * 60)) / (1000 * 60));
    seconds = Math.floor((diff % (1000 * 60)) / 1000);
  }
  else {
      days = hours = minutes = seconds = 0;
  }

// display
  document.getElementById("timer").innerHTML =
    "<div class=\"days\"> \
  <div class=\"numbers\">" + days + "</div>days</div> \
<div class=\"hours\"> \
  <div class=\"numbers\">" + hours + "</div>hours</div> \
<div class=\"minutes\"> \
  <div class=\"numbers\">" + minutes + "</div>minutes</div> \
<div class=\"seconds\"> \
  <div class=\"numbers\">" + seconds + "</div>seconds</div> \
</div>";

}, 1000);