JavaScript 实现按钮控制计时器启停功能

<span style="font-size:18px;"><body> 
 小闹钟: <span id="nz"></span> 
 <input type="button" value="开始计时" onclick="nzbg(this)"> 
 <input type="button" value="结束计时" onclick="nzbg(this)"> 
 <script type="text/javascript"> 
  function nzbg(obj) 
  { 
   var a = document.getElementById("nz"); 
   time = new Date().toLocaleString(); 
   a.innerHTML = time; 
   if (obj.value == "开始计时") 
   { 
    nz_bg = window.setInterval("nzbg()",1000); 
   } 
   if (obj.value == "结束计时") 
   { 
    nz_bg = window.clearInterval(nz_bg); 
   } 
  } 
 </script> 
 </body></span>