js执行顺序的问题

情况一
<head>
    <title>登录失败</title>
   
</head>
<body>
    <center>
        <img src="images/loading.gif" />
        <h1 style="color: RosyBrown">
            修改失败了。<span id="faileInfor"></span>秒后 跳转回修改界面</h1>
        <img src="images/sorry.jpg" />
    </center>
</body>
 <script type="text/javascript">
       
        var i = 5;
        var myspan = document.getElementById("faileInfor");
        function chang() { 
            myspan.innerText = i;
            if (--i == 0) {
                window.clearInterval(mytime);
                window.location.href = "Modify.aspx?";
            }
        }        // 启用一个定时器
        var mytime = window.setInterval("chang()", 1000);
    </script>
</html>
情况二
<head>
    <title>登录失败</title>
    <script type="text/javascript">
       
        var i = 5;
        var myspan = document.getElementById("faileInfor");
        function chang() { 
            myspan.innerText = i;
            if (--i == 0) {
                window.clearInterval(mytime);
                window.location.href = "Modify.aspx?";
            }
        }        // 启用一个定时器
        var mytime = window.setInterval("chang()", 1000);
    </script>
</head>
<body>
    <center>
        <img src="images/loading.gif" />
        <h1 style="color: RosyBrown">
            修改失败了。<span id="faileInfor"></span>秒后 跳转回修改界面</h1>
        <img src="images/sorry.jpg" />
    </center>
</body></html>
 情况一实现了功能  
经测试JS的执行顺序是先执行<head>标签中的JS 再执行<body>后的JS
  感觉情况二的定时器没有工作  
   不理解