js实现倒计时功能

<script>
    var longTime = 0;
  var t;
  function writeTime()
  {
      var timeLong = longTime;
      var days= parseInt(timeLong/(60 * 60 * 24));
         var hour = parseInt(0);
         var minute = parseInt(0);
         var second = parseInt(0);
      var s ="";
      if(days>0){
          s = days+"day ";
          timeLong = timeLong - days * (60 * 60 * 24);
      }
      hour = Math.floor(timeLong / (60 * 60));
     if (hour >= 0) {
     timeLong = timeLong - hour * (60 * 60);
     s = s+(hour<10?"0":"")+hour+ ":";
     }
     minute = Math.floor(timeLong / 60);
     if (minute >= 0) {
     timeLong = timeLong - minute * 60;
     s = s+(minute<10?"0":"")+minute+ ":";
     }
     sec = timeLong % 60;
     s = s+(sec<10?"0":"")+sec;
      document.all("spanTime").innerHTML=s;
      
      if(longTime>0){
          longTime--;
          t = setInterval("writeTime()", 1000);
          clearInterval(t-1);
      }else{
          clearInterval(t);
      }
  }

  function reduceTime(paramTime){
      longTime = paramTime;
      if(longTime>0)
      writeTime();
  }
</script>
<div style="color:black;font-size:9pt" id="spanTime">00:00:00</div>
<input type="button" name="dyTime" value="Invoke" onclick="reduceTime(3)">