эти 2 функции рабочии
<script type="text/javascript">
var mDate = new Date();
var s = mDate.getSeconds();
var m = mDate.getMinutes();
var h = mDate.getHours();
var stopT = 0;
function t()
{
var sec = 0;
var myDate = new Date();
sec = myDate.getHours()*3600 + myDate.getMinutes()*60 + myDate.getSeconds() - h*3600 - m*60 - s;
if(sec < 59)
f.tSec.value = sec;
if(sec >= 59)
{
f.tSec.value = sec % 60;
f.tMin.value = parseInt(sec / 60);
}
stopT = setTimeout(t, 250);
}
var ms = 0;
var s = 0;
var stopT2 = 0;
function t2()
{
if(ms >= 9)
{
ms = 0;
s += 1;
}
else
ms += 1;
f.tSec.value = ms;
f.tMin.value = s;
stopT2 = setTimeout(t2, 100);
}
function start()
{
if(f.rad[0].checked) t();
if(f.rad[1].checked) t2();
}
</script>
</head>
<body>
<form name="f">
мин<input type="text" name="tMin" VALUE="0"/>
сек<input type="text" name="tSec" VALUE="0"/><br/>
<input type="button" value="Запустить секундомер" onclick="start()"/><br/>
<INPUT TYPE="RADIO" NAME="rad" CHECKED>шаг 1 секунда<br />
<INPUT TYPE="RADIO" NAME="rad">шаг 0.1 секунда<br />
<input type="RESET" value="Сброс значений"/>
</form>
</body>