Coordinated Universal Time

Coordinated Universal Time or UTC, also known as civil time, is the reference time zone from which all other time zones around the world are calculated. It is the successor of Greenwich Mean Time, abbreviated as GMT, and is still colloquially called GMT sometimes. The new name was coined to eliminate having the name of a specific location in an international standard. UTC bases time measurement on atomic standards rather than GMT's celestial ones.

Because the rotation of the Earth slows down, GMT lags behind atomic time, measured by atomic clocks. UTC is synchronized to the day and night of UT1; leap seconds are added (or removed) at the end of either June or December whenever necessary. The issuing of leap seconds is determined by the International Earth Rotation Service, based on their measurements of the Earth's rotation.

"UTC" is not a real abbreviation; it is a variant of Universal Time, abbreviated UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC".

True high-precision UTC times can only be determined after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus[?]. This is done under the auspices of the International Bureau of Weights and Measures (BIPM). However, atomic clocks are so accurate that only the most precise time computations need to use these corrections, and most time service users use atomic clocks that have been previously referenced to UTC to estimate UTC times.

UTC presents problems for computer systems such as Unix which store time as the number of seconds from a reference time. Because of leap seconds, it is impossible to determine the representation of a future date, because the number of leap seconds included in that date is unknown.

UTC is the time system used for many Internet and World Wide Web standards. In particular, the Network Time Protocol is designed as a way of distributing UTC time over the Internet.

The UTC time zone is sometimes denoted by the letter 'Z' for military purposes. Since the NATO phonetic alphabet word for 'Z' is "Zulu", UTC is sometimes known as Zulu time.

Wikipedia's own server uses Coordinated Universal Time as the basis for its Wikipedia article updates list.

See also

• sidereal time
• Time zone

External Links

• BIPM UTC/TAI Time Server (http://www.bipm.fr/enus/5_Scientific/c_time/time_server)
• thetimeNOW - Current time in all time zones (http://www.thetimenow.com/)
• The official U.S. time (http://www.time.gov/)
• United States Naval Observatory - What is Universal Time? (http://aa.usno.navy.mil/faq/docs/UT)
• International Earth Rotation Service Leap Second Updates (http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat)

• W3C Specification about UTC Date and Time (http://www.w3.org/TR/NOTE-datetime) and IETF Internet standard RFC 3339 (http://www.ietf.org/rfc/rfc3339.txt).
• UTC Tag for web pages (http://journeytoforever.org/energiaweb/dttag.htm)