I have been using the UdpNtpClient code from http://arduino.cc/en/Tutorial/UdpNtpClient
successfully for months. Earlier this week, however, my Arduino Ethernet stopped connecting to the time server. I determined that the IP address used in the example no longer worked for me. Using an alternate site fixed the issue.
Original that no longer works for me: IPAddress timeServer(192, 43, 244, 18);
New address that works for me: IPAddress timeServer (198, 60, 73,
There is a list of servers at http://tf.nist.gov/tf-cgi/servers.cgi