That´s quite easy - the server is not sending the real countdown to the client. It is sending only a kind of "trigger" signal. The countdown time is calculated on each client based on the system time. So please make sure, that all devices are in the same timezone and that they are totally in sync. The easiest way to achieve that, is to enable "Set date and time automatically" in System Preferences - Date & Time.
Customer support service by UserEcho