<input type="text" id="timetext" value="00:00:00" readonly>
<button type="button" onclick="start()">开始</button> 
<button type="button" onclick="stop()">暂停</button> 
<button type="button" onclick="Reset()">重置</button>

<script>
  var hour,minute,second;//时 分 秒
  hour=minute=second=0;//初始化
  var millisecond=0;//毫秒
  var int;
  function Reset()//重置
  {
   window.clearInterval(int);
   millisecond=hour=minute=second=0;
   document.getElementById('timetext').value='00:00:00:000';
  }
 
  function start()//开始
  {
   int=setInterval(timer,50);
  }
 
  function timer()//计时
  {
   millisecond=millisecond+50;
   if(millisecond>=1000)
   {
    millisecond=0;
    second=second+1;
   }
   if(second>=60)
   {
    second=0;
    minute=minute+1;
   }
 
   if(minute>=60)
   {
    minute=0;
    hour=hour+1;
   }
   document.getElementById('timetext').value=toDub(hour)+':'+toDub(minute)+':'+toDub(second);
 
  }

  function stop()//暂停
  {
   window.clearInterval(int);
  }
  //补零
  function toDub(n){
    return n<10"0"+n:""+n;
  }
</script>