Civil time

In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC) or from Greenwich Mean Time (GMT), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks, and was adopted in 1972. Older systems use telescope observations.

In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight, but the two Nautical Almanac Offices did not accept it until 1952.

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Time Scales

Time measurement and standards

Chronometry Orders of magnitude Metrology

International standards	UTC UTC offset UT ΔT DUT1 IERS ISO 31-1 ISO 8601 TAI 12-hour clock 24-hour clock Barycentric Coordinate Time Barycentric Dynamical Time Civil time Daylight saving time Geocentric Coordinate Time International Date Line Leap second New Earth Time Solar time Terrestrial Time Time zone

Obsolete standards	Ephemeris time Greenwich Mean Time Prime meridian

Time in physics	Absolute time and space Spacetime Chronon Continuous time Coordinate time Cosmological decade Discrete time Planck epoch Planck time Proper time Theory of relativity Time dilation Gravitational time dilation Time domain T-symmetry

Horology	Clock Astrarium Atomic clock Complication History of timekeeping devices Hourglass Marine chronometer Marine sandglass Radio clock Watch Water clock Sundial Dialing scales Equation of time History of sundials Sundial markup schema

Calendar	Astronomical Dominical letter Epact Equinox Gregorian Hebrew Hindu Intercalation Islamic Julian Leap year Lunar Lunisolar Solar Solstice Tropical year Weekday determination Weekday names

Archaeology and geology	Dating methodologies Geologic time scale International Commission on Stratigraphy

Astronomical chronology	Galactic year Nuclear timescale Precession Sidereal time

Units of time	Century Day Decade Fortnight Hour Jiffy Lustrum Millennium Minute Moment Month Paksha Saeculum Second Shake Tide Week Year

Related topics	Chronology Duration Mental chronometry Metric time System time Time value of money Timekeeper

Time

Key concepts

Measurement and
standards

Chronometry	UTC UT TAI Unit of time Planck time Second Minute Hour Day Week Month Year Decade Century Millennium Tropical year Solar year Sidereal year

Measurement systems	Time zone 12-hour clock 24-hour clock Daylight saving time New Earth Time Solar time Sidereal time Metric time Decimal time Hexadecimal time

Calendars	Gregorian Julian Hebrew Islamic Lunar Solar Hijri Mayan Intercalation Leap second Leap year

Clocks

Chronology
History

Religion
Mythology

Philosophy of time

Human experience
and use of time

Time in

Geology	Geological time age chron eon epoch era period Geochronology Geological history of Earth

Physics	Absolute time and space Arrow of time Chronon Coordinate time Imaginary time Planck epoch Planck time Proper time Rate Spacetime Theory of relativity Time dilation gravitational Time domain T-symmetry

other subject areas	Chronobiology Circadian rhythms Dating methodologies in archaeology Time geography

Civil time

See also

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