关于easyui的问题

主页面是
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="main.aspx.cs" Inherits="main" %>
 
<!DOCTYPE html>
 
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
<link rel="stylesheet" href="css/StyleSheet.css" type="text/css" />
<link rel="stylesheet" href="jquery-easyui-1.3.4/themes/default/easyui.css" type="text/css" />
<link rel="stylesheet" href="jquery-easyui-1.3.4/themes/icon.css" type="text/css" />
<script type="text/javascript" src="jquery-easyui-1.3.4/jquery.min.js"></script>
<script type="text/javascript" src="jquery-easyui-1.3.4/jquery.easyui.min.js"></script>
</head>
<body class="easyui-layout">
    <div id="west_div" data-options="region:'west',href:'west.aspx',split:'true',title:'菜单'" style="width:200px;" ></div>   
    <div data-options="region:'center',href:'center.aspx'" ></div>
    <div data-options="region:'north',href:'north.aspx'" style="height:150px"></div>
</body>  
</html>
里面的north.aspx页面是
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="north.aspx.cs" Inherits="west" %>
 
<!DOCTYPE html>
 
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
<link rel="stylesheet" href="css/StyleSheet.css" type="text/css" />
<%--<script type="text/javascript" src="js/time.js"></script>--%>
</head>
<body class="northbody">
    <script type="text/javascript">
    //定义一个tick函数，以获取系统的时间
    function tick()
    {
        var year,month,day,hours,minutes,seconds,ap;
        var intYear,intMonth,intDay,intHours,intMinutes,intSeconds;
        var today;
        today=new Date();
        intYear=today.getFullYear();
        intMonth=today.getMonth()+1;
        intDay=today.getDate();
        intHours=today.getHours();
        intMinutes=today.getMinutes();
        intSeconds=today.getSeconds();
        //获取系统时间的小时数
        if(intHours==0)
        {
            hours=intHours+":";
            ap="凌晨";
        }
        else if(intHours<12)
        {
            hours=intHours+":";
            ap="早晨";
        }
        else if(intHours==12)
        {
            hours=intHours+":";
            ap="中午";
        }
        else
        {
            hours=intHours+":";
            ap="下午";
        }
        //获取系统时间的分数
        if(intMinutes<10)
        {
            minutes="0"+intMinutes+":";
        }
        else
        {
            minutes=intMinutes+":";
        }
        //获取系统时间的秒数
        if(intSeconds<10)
        {
            seconds="0"+intSeconds+" ";
        }
        else
        {
            seconds=intSeconds+" ";
        }
        timeString=intYear+'年'+intMonth+'月'+intDay+'日'+hours+minutes+seconds+ap;
        lab_time.innerHTML=timeString;
        //每隔0.1秒钟执行一次tick函数
        window.setTimeout("tick()",100);
    }
            window.onload=tick;
    </script>
    <div class="datebar">
        <table>
            <tr>
                <td>现在是：</td>
                <td><asp:Label ID="lab_time" runat="server"></asp:Label></td>
            </tr>
        </table>
    </div>
    <div class="userbar">
        <table>
            <tr>
                <td>欢迎您：</td>
                <td style="width:200px"><asp:Label ID="Label2" runat="server" Text=""></asp:Label></td>
                <td class="td_btn"><a>重新登录</a></td>
                <td class="td_btn"><a>修改密码</a></td>
                <td class="td_btn"><a>退出系统</a></td>
            </tr>
        </table>
    </div>
</body>
</html>问题是，单独运行子页面可以显示时间
如果是主页面的话，用浏览器（firefox）打开的话有时刷新就可以看见（IE不行...）
但主页面F5运行的时候firefox也看不到了...为什么呢...