JavaScript实现页面日期倒计时功能

<!DOCTYPE html>
<html>
<head>
 <meta charset="utf-8">
 <style type="text/css">
  body
  {
   color:red;
   text-align: center;
  }
  .one{
   color:black;
   text-align:center;
  }
 </style>
</head>
<body>
<p>
 日期倒计时<br>
 请输入目标日期并点击按钮，实现目标日期的倒计时！
</p>
<form name="form1">
 <input type="text" id="a" size=1 name="a">年
 <input type="text" id="b" size=1 name="b">月
 <input type="text" id="c" size=1 name="c">日
 <input type="text" id="d" size=1 name="d">时
 <input type="text" id="e" size=1 name="e">分
 <input type="text" id="f" size=1 name="f">秒
 <br>

 <div id="time" value="时间"></div> 
 <script>
  function getdate(clock){
   var now=new Date();
   var y=now.getFullYear()
   var m=now.getMonth()
   var d=now.getDate()

   var day=now.getDay()
   var h=now.getHours()
   var minu=now.getMinutes()
   var s=now.getSeconds()

   m+=1;
   var array=new Array("星期日","星期一","星期二","星期三","星期四","星期五","星期六")
   var nowtime=y+"年"+m+"月"+d+"日 "+array[day]+" "+h+"时"+minu+"分"+s+"秒";
   clock.innerHTML="当前时间："+nowtime; 
  }
  window.onload=function(){  
  window.setInterval("getdate(time)",1000)
  }
 </script>

 <input type="button" value="倒计时" onclick="leftTime()">:
 距离目标日期还有：
 <input type="text" id="cha" >
 <script> 
 var flag=1;var t;
 function leftTime(){ 

 if(flag==0){flag=1;}

 year=document.getElementById("a").value;
 month=document.getElementById("b").value;
 day=document.getElementById("c").value;
 hour=document.getElementById("d").value;
 minute=document.getElementById("e").value;
 second=document.getElementById("f").value;

 var leftTime = (new Date(year,month-1,day,hour,minute,second)) - (new Date()); 
 //计算剩余的毫秒数 
 if(leftTime>0){
 var days = parseInt(leftTime/1000/60/60/24,10); //计算剩余的天数 
 var hours = parseInt(leftTime/1000/60/60%24,10); //计算剩余的小时 
 var minutes = parseInt(leftTime/1000/60%60,10);//计算剩余的分钟 
 var seconds = parseInt(leftTime/1000%60,10);//计算剩余的秒数 
  document.getElementById("cha").value=days+"天" + hours+"小时" + minutes+"分"+seconds+"秒";
 }
 else{
  document.getElementById("cha").value="0天0小时0分0秒";
  alert("主人，时间到啦！")
  flag=0;clearInterval(t);
 }
 if(flag==1){
 t=setInterval("leftTime()",1000);
 flag=2;
 }
 } 
 </script>
</form> 
</body>
</html>