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Because the rotation of the Earth slows down, GMT lags behind atomic time, measured by atomic clocks. UTC is kept within 0.9 s of UT1; leap seconds are added (or, theoretically, subtracted) on June 30 or December 31 as necessary. To date -- the first being in 1972 -- all such adjustments have been positive, adding a leap second at 23:59:60. The issuing of leap seconds is determined by the International Earth Rotation and Reference Systems 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" (temps universel coordonné).
| Pacific Standard Time | UTC -8 |
|---|---|
| Mountain Standard Time | UTC -7 |
| Central Standard Time | UTC -6 |
| Eastern Standard Time | UTC -5 |
| Atlantic Standard Time | UTC -4 |
| Greenwich Mean Time | UTC |
| Central European Time | UTC +1 |
| Eastern European Time | UTC +2 |
| Moscow Time | UTC +3 |
| Indian Standard Time | UTC +5:30 |
| Australian Western Standard Time ( China) | UTC +8 |
| JapanJapan (, Nippon/Nihon literally "the origin of the sun") is a country in East Asia situated on a chain of islands east of the Asian continent on the western edge of the Pacific Ocean. The largest of these islands are, from north to south, Hokkaido , Honsh/ KoreaKorea is a formerly unified country, situated on the Korean Peninsula in north East Asia adjacent to China in the west and Russia in the north. When World War II ended in 1945, the country was divided into two: the Republic of Korea ( South Korea) and the Standard Time | UTC +9 |
| Australian Eastern Standard Time | UTC +10 |
International standard UTC time can only be determined to the highest precision 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 bureau x. This is done under the auspices of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) (BIPM). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds.
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 – NTP is designed as a way of dynamically distributing UTC time over the Internet.
There are some classes of software UTC clocks:
As indicated in the standards, it is convenient to include the UTC date too.
The UTC time zone is sometimes denoted by the letter Z since the equivalent nautical time zone (GMT) has been denoted by Z since about 1950, and by a "zone description" of zero hours since 1920. See Time zone#History. Since the NATO phonetic alphabet and radio-amateur word for Z is "Zulu", UTC is sometimes known as Zulu time.