为什么以下代码,在页面刷新时有效,页面关闭却无效?!

<html>
<head>
<title>JavaScript变量,运算符和表达式的例子</title>
</head>
<body onunload="stay()";>
<center>
<font face=楷体 size=5 color=blue>关闭该窗口,看看会发生什么?</font>
</center>
<script language=javascript>
<!--
pageopen=new Date();
function stay(){
pageclose=new Date();
hours=(pageclose.getHours()-pageopen.getHours());
minutes=(pageclose.getMinutes()-pageopen.getMinutes());
seconds=(pageclose.getSeconds()-pageopen.getSeconds());
time=hours*3600+minutes*60+seconds;
alert("您在页面停留的时间为:"+time+"秒钟");}
//-->
</script>
</body></html>