60秒定时减少

<!DOCTYPE html>
<html>

   <head>
       <meta charset="UTF-8">
       <title></title>
       <style type="text/css">
           div {
               200px;
               height: 200px;
               border: 1px solid black;
               margin: 50px auto;
               text-align: center;
               line-height: 200px;
               font-size: 50px;
           }
       </style>
   </head>

   <body>
       <div id="id"></div>
   </body>
   <script type="text/javascript">
       var id = document.all.id;

       var a = 61;

       var n = setInterval(function() {
           a--;
           id.innerHTML = a + "s";
           if(a == 0) {

               clearInterval(n)
               id.innerHTML=('时间到了')

           }
       }, 1000);
   </script>

</html>