Puoi fare cosi.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="javascript">
function viewDate() {
 var aDate = new Date();
 document.getElementById('date').innerHTML = aDate.toLocaleString();
} // function viewDate()
</script>
</head>
<body>
<div id="date"></div>


<script language="JavaScript" type="text/javascript">
<!--
setInterval("viewDate()",1000);
//-->
</script>

</body>
</html>