NTP Internals and background
NTP operates on UDP port 123, so you must ensure that this port is left open on your firewall. NTP is one of the oldest Internet protocols still in regular use today. The Network Time Protocol allows client computers to synchronise to an accurate time reference over the Internet.
NTP can synchronise networks across the Internet or a Local/Wide Area Network (LAN/WAN) it synchronises time with reference to a reliable clock source. This source could be relative such as a computer's internal clock or the time on a wrist-watch or absolute such as time from an atomic clock.