You can use iframe and source a site with time. You can also insert a time widget. http://www.google.com/search?q=time+widget
HTML itself is for displaying things, not so much for processing anything like getting the server/computer time. Try one of the more "programming-like" suggestions like PHP or JavaScript. I think PHP has a very simple solution if your server has the option enabled. http://ca2.php.net/manual/en/function.date.php