Awesome Javasript Analysis:Changing the background & rotating differently Banners every day

Belows Javacript is the fourm index of http://forums.winglin.net/kenixkwok I see the background and the banners every day can automatcily change, but I do not really understand how exaclty belwos script works in order to to let background and teh banners every day can automatcily change, instead of manual change

**their backorund is based on
http://forums.winglin.net/kenixkwok/pictures/bg_1.jpg
http://forums.winglin.net/kenixkwok/pictures/bg_2.jpg
...

***their banners is is based on
http://forums.winglin.net/kenixkwok pictures/banner_1.jpg
http://forums.winglin.net/kenixkwok pictures/banner_2.jpg
.....

I also want to achive:" teh index page automtcilly change the Backround pix and teh banners pix is auto rotating every 4 banners like them, that is too awesome...(^_^)

i see this line :var interval = 5;
var base_URL = "pictures/banner_";
var banners = [42, 41, 'a', 30]; // substitute this line

is abvoe 42, 41, 'a', 30 is automatcilly created by the javascript??

how exactly belwos, can u give me soem explanation I donto understand at all

Does the 2 scheme define is 2 style that automactilly change every day?

-------original javascript------