Instead of initializing timer via plain Javascript (which will return the client-time, i.e., the time on your client's computer), you should instead rely on server-time, i.e., the time on your server. This will standardize the timing for all your players.
So, upon visiting the countdown page, your Javascript counter will be initialized with the server's time. You could use AJAX or some hack-y PHP to do this. For this task, start-up is the only time JS needs something from the server; afterwards, the "countdown" effect will be managed by JS. Of course, this will not be uber-synced but that error is pretty forgivable. Even if the user refreshes with a different local timezone setting, the JS will just re-initialize from server time--no problems!