Network Time Protocol; IP protocol that synchronizes computer time by sending and receiving packets via UDP port 123. The current version of the standard, version 4, was adopted in June 2010 according to the RFC page and is backwards compatible with version 3.
The Home of NTP: http://www.ntp.org
Wikipedia article: http://en.wikipedia.org/wiki/Network_Time_Protocol
National Institute of Time (NIST) Time Service: http://www.nist.gov/pml/div688/grp40/its.cfm