var how_many_ads = 4;
var now = new Date();
var sec = now.getSeconds();
var num = sec % how_many_ads;
var ad_content=new Array(); 

//-->

ad_content[0] ='<IFRAME FRAMEBORDER=0 MARGINWIDTH=0 MARGINHEIGHT=0 SCROLLING=NO WIDTH=728 HEIGHT=90 SRC="http://www.backchina.com/ads/html/dishnetwork_728x90.htm"></IFRAME>' 
ad_content[1] ='<IFRAME FRAMEBORDER=0 MARGINWIDTH=0 MARGINHEIGHT=0 SCROLLING=NO WIDTH=728 HEIGHT=90 SRC="http://www.backchina.com/ads/html/dishnetwork_728x90.htm"></IFRAME>'
ad_content[2] ='<IFRAME FRAMEBORDER=0 MARGINWIDTH=0 MARGINHEIGHT=0 SCROLLING=NO WIDTH=728 HEIGHT=90 SRC="http://pic.backchina.com/banners/xoom/money_728x90.php"></IFRAME>'
ad_content[3] ='<IFRAME FRAMEBORDER=0 MARGINWIDTH=0 MARGINHEIGHT=0 SCROLLING=NO WIDTH=728 HEIGHT=90 SRC="http://news.backchina.com/ads/homepage_728x90_flash.htm"></IFRAME>'

document.write( ad_content[num]  )