怎么在网页中制作倒计时

在代码的body部分加上以下代码

<SCRIPT language=javascript><!--
BirthDay=new Date("january 01,2000");//改成你的计时日期
today=new Date();
timeold=(today.getTime()-BirthDay.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);
//-->
</SCRIPT><SCRIPT language=javascript><!--
document.write("新世纪已经进去了"+daysold+"天"+hrsold+"小时"+minsold+"分"+seconds+"秒了!");
//-->
</SCRIPT>