Vedi se questo va bene.
Lo usai io alcuni mesi fa.


<head>
<style>
#corpo {
width:450px;
position: relative;
text-align: center;
margin-left: auto;
margin-right: auto;
margin-top: 15px;
height:auto !important;
height:0;
}
form {
text-align: center;
margin: auto;
border: none;
}

input {
text-align: center;
margin: auto;
border: none;
}

.T1 {font-size:13px; font-family:Verdana, Arial, Helvetica; text-decoration:none}
a {text-decoration:none}
.nobordo {border: none; text-align: center; font-family: Arial; font-size: 16 px; font-weight: bold;}
</style>

<title></title>


<script language="JavaScript" type="text/JavaScript">
<!--
function getTime() {
now = new Date();
y2k = new Date("Mar 16 2005 00:00:00");
days = (y2k - now) / 1000 / 60 / 60 / 24;
daysRound = Math.floor(days);
hours = (y2k - now) / 1000 / 60 / 60 - (24 * daysRound);
hoursRound = Math.floor(hours);
minutes = (y2k - now) / 1000 /60 - (24 * 60 * daysRound) - (60 * hoursRound);
minutesRound = Math.floor(minutes);
seconds = (y2k - now) / 1000 - (24 * 60 * 60 * daysRound) - (60 * 60 * hoursRound) - (60 * minutesRound);
secondsRound = Math.round(seconds);
sec = "s ";
min = "m ";
hr = "h ";
dy = "g ";
document.theTimer.theTime.value = daysRound + dy + hoursRound + hr + minutesRound + min + secondsRound + sec + "all'inizio del corso";
newtime = window.setTimeout("getTime();", 1000);
}
function ApriCentrato(pagina,nome,w,h){
var winl = Math.floor((screen.width - w) / 2);
var wint = Math.floor((screen.height - h) / 2);
window.open(pagina,nome,'toolbar=no,status=no,scro llbars=no,left='+winl+',top='+wint+',width='+w+',h eight='+ h);
}

//-->
</script>

<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
</head>
<body onLoad="getTime()" bgcolor="#FFFFFF" text="#000000" link="#000000" vlink="#000000" alink="#000000">



</p>
<table width="306" border="0" align="center">
<tr>
<td width="300"><form name="theTimer" id="theTimer">
<input type="text" name="theTime" size="45" class="nobordo" border="0" />
</form></td>
</tr>
</table>
<p align="center"> </p>


<p align="center"></p>


</body>
</html>