Så här kanske:
<script type="text/javascript">
<!--
function calc(){
var f = new Date( "mar 18 2003" );
var t = new Date();
var diff = f.valueOf() - t.valueOf();
var days = diff / 1000 / 60 / 60 / 24;
var hours = days * 24 - Math.floor( days ) * 24;
var mins = hours * 60 - Math.floor( hours ) * 60;
document.write( "Det är " + Math.floor( days ) + " dagar, " + Math.floor( hours ) + " timmar och " + Math.floor( mins ) + " minuter..." );
};
//-->
</script>
...
<body>
<script type="text/javascript">calc();</script>
</body>
:)