用JS怎么写一个奥运倒计时，偶不会

[CODE]<!--done-->
<div class="newsItem">
<p align="center">
<b><align=center><font color=#FF0000>北京奥运倒计时</font></b><font color="#FF0000">
</font>

<span id="span_dt_dt" style=''''align=center no:1px solid black;background-color:#FFFFFF'''' ></span>

<font color="#FF0000">

<SCRIPT language=javascript>
<!--
//document.write("");

function show_date_time(){
window.setTimeout("show_date_time()", 1000);
BirthDay=new Date("8-8-2008");//这个日期是可以修改的
today=new Date();
timeold=(BirthDay.getTime()-today.getTime());
sectimeold=timeold/1000
secondsold=Math.floor(sectimeold);
msPerDay=24*60*60*1000
e_daysold=timeold/msPerDay
daysold=Math.floor(e_daysold);
e_hrsold=(e_daysold-daysold)*24;
hrsold=Math.floor(e_hrsold);
e_minsold=(e_hrsold-hrsold)*60;
minsold=Math.floor((e_hrsold-hrsold)*60);
seconds=Math.floor((e_minsold-minsold)*60);
span_dt_dt.innerHTML="<b><align=center><font color=00CC00>"+daysold+"天"+hrsold+"小时"+minsold+"分"+seconds+"秒"+"</b><br></font>" ;

}

show_date_time();

//-->
</SCRIPT>
</font>[/CODE]