登陆成功后自动计算秒数执行跳转

<script language = "javascript" type = "text/javascript"> 
function $(id){ 
return document.getElementById(id); 
} 

function checkUser(){ 
if($("u1").value == "hello" && $("p1").value == "123"){ 
return true; 
}else{ 
return false; 
} 
} 
</script> 
<form action = "ok.html"> 
<div style = "width:300px;height:200px;border:1px solid red;background-color:pink"> 
<p/> 
U:<input id = "u1" type = "text" value = "" /> <br><br> 
P:<input id = "p1" type = "text" value = "" /> <br><br> 
<input type = "submit" onclick = "return checkUser()" value = "登陆" style = "margin-left:20px"/> 
</div> 
</form>

<script language = "javascript" type = "text/javascript"> 
setTimeout("javascript:clearInterval(mytime);window.open('manage.html')",5000); 
function changeSec(){ 
document.getElementById("myspan").innerText = 
parseInt(document.getElementById("myspan").innerText) - 1; 
} 
var mytime = setInterval("changeSec()",1000); 
</script> 
登陆成功 <span id = "myspan">5</span> 秒钟后自动跳转...

welcome