Why do you use AJAX to update countdown?

Using AJAX for this would give you MUCH less accuracy than plain-old JavaScript with time delta of user time to server time:

If you just need to time sync - you could receive server time once (remembering user time, when you sent request for server time), then just compensate user time with that value.

In PSEUDO-JavaScript (client-side):

  // do this once:
  var user_time = (new Date()).getTime();
  ajax.call('/server-time-in-sec-since-epoch', 
    callback: 
      delta_time = recvd_server_time - user_time;
  )
  
  // then at any given second real server time is:
  var current_server_time = (new Date()).getTime() + delta_time;
  // no need for ajax calls

For more accuracy you should divide delta_time by 2 (since it's round-trip).