function startTime(){
     var today=new Date();
     var h=today.getHours();
     var m=today.getMinutes();
     var s=today.getSeconds();// 在小于10的数字前加一个‘0’
     m=checkTime(m);
     s=checkTime(s);
     document.getElementById('txt').innerHTML=h+":"+m+":"+s;
     t=setTimeout(function(){startTime()},500);
 }
 function checkTime(i){
     if (i<10){
         i="0" + i;
     }
     return i;
 }
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Date（日期） 对象</title>
</head>
<body>
  
</body>
</html>
<script>
var today = new Date();
console.log(today)

console.log(today.getTime())

console.log(today.getFullYear())
</script>