javascript倒计时执行文件

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
 <HEAD>
  <TITLE> New Document </TITLE>
  <META NAME="Generator" CONTENT="EditPlus">
  <META NAME="Author" CONTENT="">
  <META NAME="Keywords" CONTENT="">
  <META NAME="Description" CONTENT="">
 </HEAD>
 <BODY>
  <script language="JavaScript"> 
<!--
var seconds = 10;
onload = function()
{
  window.setInterval(redirection, 1000);
}
function redirection()
{
  if (seconds <= 0)
  {
    window.clearInterval();
    return;
  }
 
  seconds --;
  document.getElementById('plan_interval').innerHTML = seconds;
 
  if (seconds == 0)
  {
    //window.clearInterval();
alert('aaa');
document.getElementById('plan_num').innerHTML = document.getElementById('plan_num').innerHTML+1;
seconds = 10;
  }
}
//-->
</script> 
<TR class="row" > 
<th  class="tCenter red" align="center">正在执行  [ <font color="#FF0000">导入留言</font> ] [ 已执行 <span id="plan_num">3154</span>  &nbsp;次 ]  [ 离下次执行时间还有 <span id="plan_interval">360</span>&nbsp;秒 ]</th> 
</TR> 
<div> 
<iframe src="/cron.php" frameborder="0" width="100%" height="400" scrolling="no" id="plan"></iframe> 
</div> 
 </BODY>
</HTML>