The Network Time Protocol (NTP) is used to synchronize the time of a
computer client or server to another server or reference time source,
such as a radio or satellite receiver or modem.

It provides client accuracies typically within a millisecond on LANs
and up to a few tens of milliseconds on WANs relative to a primary
server synchronized to Coordinated Universal Time (UTC) via a Global
Positioning Service (GPS) receiver, for example.

Typical NTP configurations utilize multiple redundant servers and diverse
network paths, in order to achieve high accuracy and reliability.

Some configurations include cryptographic authentication to prevent
accidental or malicious protocol attacks.

See homepage for more infos:

WWW: http://www.eecis.udel.edu/~ntp/