<form>

<input type="text" id="count" /><button onclick="i">计时</button>

<button onclick="j">停止</button>

</form>

<script type="text/javascript">

  var num=0;

  function startCount() {

      num+=1;

    document.getElementById('count').value=num;

  }

  var i=setInterval("startCount()",1000)；

   var j=setTimeout(clearInterval("i"),3000);

</script>