<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>定时器实现倒计时</title>
  <style>
     body {
       font-size: 30px;
       text-align: center;
       color: red;
     }
  </style>

  <script>
    window.onload = function(){
      var demo = document.getElementById("demo");
      var endtime = new Date("2020/02/04 12:00:00"); //最终时间
      setInterval(clock, 1000); //开启定时器
      function clock() {
        var now = new Date(); //当前时间
        var second = parseInt((endtime.getTime() - now.getTime()) / 1000); //需要倒计时的时间
        var day = parseInt(second / 3600 / 24); //天数
        var hour = parseInt(second / 3600 % 24); //时
        var minute = parseInt(second / 60 % 60); //分
        var s = parseInt(second % 60); //秒
        
        // 三目运算添加10以下数字前面的 0 
        day < 10 ? day = "0"+day : day;
        hour < 10 ? hour = "0"+hour : hour;
        minute < 10 ? minute = "0"+minute : minute;
        s < 10 ? s = "0"+s : s;

        demo.innerHTML = "倒计时时间还剩："+day+"天 "+hour+"小时 "+minute+"分钟 "+s+"秒" 
      }
    }
  </script>
</head>
<body>
  <div id="demo">
    
  </div>
</body>
</html>