index.js

const day = document.querySelector(".day .numb");
const hour = document.querySelector(".hour .numb");
const min = document.querySelector(".min .numb");
const sec = document.querySelector(".sec .numb");
var timer = setInterval(() => {
  var currentDate = new Date().getTime();
  var launchDate = new Date("Sep 18, 2020 13:00:00").getTime();
  var duration = launchDate - currentDate;
  var days = Math.floor(duration / (1000 * 60 * 60 * 24));
  var hours = Math.floor((duration % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
  var minutes = Math.floor((duration % (1000 * 60 * 60)) / (1000 * 60));
  var seconds = Math.floor((duration % (1000 * 60)) / 1000);
  day.innerHTML = days;
  hour.innerHTML = hours;
  min.innerHTML = minutes;
  sec.innerHTML = seconds;
  if (days < 10) {
    day.innerHTML = "0" + days;
  }
  if (hours < 10) {
    hour.innerHTML = "0" + hours;
  }
  if (minutes < 10) {
    min.innerHTML = "0" + minutes;
  }
  if (seconds < 10) {
    sec.innerHTML = "0" + seconds;
  }
  if (duration < 0) {
    clearInterval(timer);
  }
}, 1000);