apart/main.js

document.addEventListener(
  "DOMContentLoaded",
  function () {
    var days = document.querySelector(".days span");
    var hour = document.querySelector(".hour");
    var min = document.querySelector(".min");
    var second = document.querySelector(".second");

    var startDate = new Date(2021, 4, 19);
    days.innerText = Math.floor((new Date() - startDate) / 86400000);
    countTime();

    function countTime() {
      let today = new Date();
      let ms = (today - startDate) % 86400000;
      hour.innerText = Math.floor(ms / 3600000);
      min.innerText = Math.floor((ms % 3600000) / 60000);
      second.innerText = Math.floor(((ms % 3600000) % 60000) / 1000);
    }

    setInterval(countTime, 1000);
  },
  false
);