Of you are talking about a page, then forget the word "form" - it has a specific meaning, and requires a different solution.
For a web page, the best way to do this is via JavaScript so it is kept to the client, and doesn't need a round trip to the server every second...
See here:
http://www.w3schools.com/js/js_timing.asp[
^] - it presents several ways to display the time!