В шапку сайта:
<script type="text/javascript" language="JavaScript">
window.onload = function(){
window.setInterval(function(){
now = new Date();
var hh = now.getHours();
var hh<10?'0'+hh:hh;
var mm = now.getMinutes()
var mm<10?'0'+mm:mm;
var clock = document.getElementById('clock');
clock.innerHTML = hh+ ':' +mm;
}, 20000);
}</script>