JavaScript counter

当页面加载以后，执行JavaScript，按秒开始计数：
JavaScript function：
<SCRIPT language="JavaScript">
<!--Timer in JavaScript
var timerform
speed=1000
function dotimer()
{
today=new Date()
slutsec=today.getSeconds()
slutmin=today.getMinutes()
sluttim=today.getHours()
sluta=(slutsec) + 60 * (slutmin) + 3600 * (sluttim)
diff=sluta - starta
tim=Math.floor(diff / 3600)
min=Math.floor((diff / 3600 - tim) * 60)
sek=Math.round((((diff / 3600 - tim) * 60) - min) * 60)
document.timerform.timer.value=tim + ':'
if(min<10)document.timerform.timer.value+='0'
document.timerform.timer.value+=min + ':'
if(sek<10)document.timerform.timer.value+='0'
document.timerform.timer.value+=sek
window.setTimeout("dotimer()",speed)
}
function Timer()
{
today=new Date()
startsek=today.getSeconds()
startmin=today.getMinutes()
starttim=today.getHours()
starta=(startsek) + 60 * (startmin) + 3600 * (starttim)
document.write('<form name=timerform><input name=timer size=7')
document.write('></form>')
dotimer()
}
// end-->
</SCRIPT>
HTML：
<table width=420>
<tr>
<td align=center>
TIMER:
<SCRIPT language="JavaScript">
Timer()
</SCRIPT>
</td>
</tr>
</table>
实现效果： http://www.javascriptsearch.com/scripts/Counters/simple_counter.html