If you need a click on any client to work with all clients, you will need to base the decision on either a survey (update the AJAX hit) or comet programming . The comet programming solution is more complex, but allows real-time pushing timer updates to all clients.
Word of Advice: If your site does not plan to make a large number of asynchronous communications, I just stick to a simple survey. You can even query the AJAX static file and update the static file via PHP (this update needs to be done atomically). This means that every AJAX survey will be very cheap.