一个关于时间显示的有关问题 _JavaScript

一个关于时间显示的问题
就是想看时间了，就单击一下，在文本框里显示时间，然后不想看了，再单击，就不显示时间了。
这个代码要怎么改才能符合上边的要求？

<html>
<head>
<script type="text/javascript">

function startTime(){

var d = new Date();
var h = d.getHours();
var m = d.getMinutes();
var s = d.getSeconds();
m = checkTime(m);
s = checkTime(s);
calculate.screen.value = h + ":" + m + ":" + s;
setTimeout('startTime()',500);

}

function checkTime(i){
if (i<10){
i="0" + i;
}
return i;
}

</script>
</head>

<body >
<form name = "calculate" action = "">
<input name = "showTime" type = "button" value = " time " onclick = "startTime()" />
<input name = "screen" type = "text" size = "32" value = "0">

</body>
</html>

------解决方案--------------------

HTML code


<script type="text/javascript">
var tid;
var b = true;
function startTime(){
  if(b) return;
  clearTimeout(tid);
  var d = new Date();
  var h = d.getHours();
  var m = d.getMinutes();
  var s = d.getSeconds();
  m = m<10?"0"+m:m;
  s = s<10?"0"+s:s;
  document.calculate.screen.value = h + ":" + m + ":" + s;
  tid = setTimeout('startTime()',1000);
}
</script>
</head>

<body >
  <form name = "calculate" action = "">
  <input name = "showTime" type = "button" value = " time " onclick = "b=!b;startTime()" />
  <input name = "screen" type = "text" size = "32" value = "0">
  </form>
</body>
</html>