javascript 随机广告代码(图片广告)

//****END 6-25***// 
var adsAry_length = tag1.length; 
var now = new Date(); 
//MOD 20090522var sec = now.getSeconds(); 
//MOD 20090522 var ad = sec % adsAry_length; 
var sec =now.getTime(); 
var ad = rand(adsAry_length) 
var url = urlArray[ad-1]; 
var banner = tag1[ad-1]; 
//DEL alert("urlArray[" +(ad-1)+"]=" + urlArray[ad-1]) 
//DEL 20090522 if(ad <= tag1.length){ 
//DEL 20090522    url=urlArray[ad]; 
//DEL 20090522    banner=tag1[ad]; 
//DEL 20090522} 
//DEL 20090522 document.write('<center>'); 
document.writeln(''); 
document.writeln('<img src="http://img.gxlcms.com/' + banner + '">'); 
//DEL 20090522 document.write('<\center>'); 
//****************************************************/ 
//rnd() 
//****************************************************/ 
function rnd() { 
    var ia=9301; 
    var ic=49297; 
    var im=233280; 
    sec = (sec*ia+ic) % im; 
    return sec/(im*1.0); 
}; 
//****************************************************/ 
//rand(number) 
//****************************************************/ 
function rand(number) { 
    return Math.ceil(rnd()*number); 
}; 
// --></script>

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]
这个里面有个不错的随机函数代码，具体的没看懂。知道的朋友可以回复下。
代码如下:
function rnd() {
var ia=9301;
var ic=49297;
var im=233280;
sec = (sec*ia+ic) % im;
return sec/(im*1.0);
};