Which Internet time-synchronization protocol normally uses UDP port 123?

The Network Time Protocol normally uses UDP port 123 for Internet time synchronization.

NTP allows computers and network devices to compare their clocks with reference time sources. Accurate clocks matter for logging, authentication, certificate validation, distributed databases, scheduled tasks, and security investigations. NTP can estimate network delay and adjust a local clock gradually rather than making an abrupt change.

The protocol was developed by David L. Mills and has evolved through several versions. Public NTP servers, organizational time servers, and local hardware references can participate in a hierarchy known as a stratum system. A device usually synchronizes with several sources so it can detect an unreliable or inconsistent response.

NTP is distinct from a time-zone setting. Time zones determine how a timestamp is displayed for a region, while NTP helps establish the underlying clock value, normally expressed relative to Coordinated Universal Time.