var how_many_ads = 2;
var now = new Date();
var sec = now.getSeconds();
var num = sec % how_many_ads;
var ad_content=new Array(); 

ad_content[0] =' <a href=http://www.6park.com target=_blank><img style=\"FLOAT: left\" hspace=5 vspace=1 align=top src=6park300.jpg width=300 height=250 border=0></a> '
ad_content[1] =' <a href=http://www.yeeyi.com target=_blank><img style=\"FLOAT: left\" hspace=5 vspace=1 align=top src=yeeyi300.jpg width=300 height=250 border=0></a> '


document.write( ad_content[num]  )