A quick google for javascript clock comes up with something:
http://www.google.com/search?hl=en&q=javascript+clock&meta=
http://www.javascript-page.com/clock.html
The only thing that would have to change is to set the date as the server date, rather than the client date.
var tDate = new Date(<%= System.currentTimeMillis() %>);
This would be set to the server time at the time of serving the page. It would probably take a few seconds to transmit/render on the client machine.
How accurate do you need this clock to be?