Network Time Protocol (NTP) is an open standard Internet protocol that utilizes standard Ethernet wiring to synchronize computers, clocks, networks and other systems to an NTP reference server. The signal can be transmitted over LAN and WAN wired networks and over various wireless systems.
There are numerous public sources of NTP available. Many universities and governmental agencies provide access to NTP servers although, because of the number of request, some of these facilities are limiting access and in some cases even charging access fees.
NTP service can be established on your internal network using a stand alone device such as the NTP100-GPS which gets time from the atomic clocks in the GPS system and generates NTP. GPS receiver cards such as the PCIe-GPS along with NTP software in a computer on the network can also provide a local NTP reference source. In many cases because of firewalls and other network constraints a local NTP server is the recommended approach and in many cases provides a more accurate and available NTP.
Accuracy of time in a typical NTP installation will depend on many factors, but it is typically better than one half‐second after one year without GPS.