We make every effort to make it as accurate as possible.
Network delays are a factor is the accuracy of the client.
The countdown time to Shuttle Launch should be within approximately one or two seconds of that of KSC Launch Control. NASA does not use this client for any actual launch operations. The Countdown applet is tied directly to live time from KSC Launch Control. The time is being distributed to the network, and experiences delays and losses, as any WAN application would. In order to keep the count distributed realtime, and strive for accuracy, the client applet recieves an update every second, as the clock ticks.