Coordinated Universal Time

World map of current time zones
Graph showing the difference DUT1 between UT1 and UTC (in seconds). Vertical segments correspond to leap seconds.

Primary time standard by which the world regulates clocks and time.

- Coordinated Universal Time

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Leap second

Screenshot of the UTC clock from https://time.gov/ during the leap second on December 31, 2016. In the U.S., the leap second took place at 18:59:60 local time on the East Coast, at 15:59:60 local time on the West Coast, and at 13:59:60 local time in Hawaii.
Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds.
Deviation of day length from SI based day with shorter days resulting from faster planetary rotation.

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation.

Time zone

Area that observes a uniform standard time for legal, commercial and social purposes.

Time zones of the world
Time zones of the world
Plaque commemorating the Railway General Time Convention of 1883 in North America
The control panel of the Time Zone Clock in front of Coventry Transport Museum
1913 time zone map of the United States, showing boundaries very different from today
World map of time zones in 1928
Difference between sun time and clock time during daylight saving time:

All time zones are defined as offsets from Coordinated Universal Time (UTC), ranging from UTC−12:00 to UTC+14:00.

Earth's rotation

Rotation of planet Earth around its own axis, as well as changes in the orientation of the rotation axis in space.

An animation of Earth's rotation around the planet's axis
This long-exposure photo of the northern night sky above the Nepali Himalayas shows the apparent paths of the stars as Earth rotates.
Earth's rotation imaged by DSCOVR EPIC on 29 May 2016, a few weeks before the solstice.
Starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory.
On a prograde planet like Earth, the stellar day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).
Plot of latitude vs tangential speed. The dashed line shows the Kennedy Space Center example. The dot-dash line denotes typical airliner cruise speed.
Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a cycle and is currently decreasing.
A simulated history of Earth's day length, depicting a resonant-stabilizing event throughout the Precambrian era.
Deviation of day length from SI-based day
An artist's rendering of the protoplanetary disk.

Atomic clocks show that a modern-day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds.

Universal Time

ERA = 2π(0.7790572732640 + 1.00273781191135448Tu) radians

Standard time zones of the world (February 2021). The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.
An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time

UT0 and UT2 soon became irrelevant due to the introduction of Coordinated Universal Time (UTC).

UTC offset

World map of current time zones

The UTC offset is the difference in hours and minutes between Coordinated Universal Time (UTC) and local solar time, at a particular place.

Solar time

Calculation of the passage of time based on the position of the Sun in the sky.

On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1-2 is a complete rotation of the Earth, but because the revolution around the Sun affects the angle at which the Sun is seen from the Earth, 1-3 is how long it takes noon to return.
The Earth's orbit around the Sun, showing its eccentricity
The equation of time—above the axis a sundial will appear fast relative to a clock showing local mean time, and below the axis a sundial will appear slow.
Sun and Moon, Nuremberg Chronicle, 1493

The difference between this corrected mean solar time and Coordinated Universal Time (UTC) determines whether a leap second is needed.

Network Time Protocol

Networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks.

NTP was designed by David L. Mills.
The U.S. Naval Observatory Alternate Master Clock at Schriever AFB (Colorado) is a stratum 0 source for NTP
Round-trip delay time δ
The NTP management protocol utility ntpq being used to query the state of a stratum 2 server.
, showing sources and activity information. Terminal window under Arch Linux

NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC).

International Atomic Time

High-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid.

NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 caesium fountain atomic clock, a civilian time standard for the United States

It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.

Daylight saving time

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An ancient water clock that lets hour lengths vary with season.
George Hudson invented modern DST, proposing it first in 1895.
DST was first implemented in the United States to conserve energy during World War I. (poster by United Cigar Stores)
Retailers generally favor DST; United Cigar Stores hailed a 1918 DST bill.
William Willett independently proposed DST in 1907 and advocated it tirelessly.
A 2001 US public service advertisement reminded people to adjust clocks.
The William Willett Memorial Sundial in Petts Wood, south London, is always on DST.

Members of the European Union conduct a coordinated change, changing all zones at the same instant, at 01:00 Coordinated Universal Time (UTC), which means that it changes at 02:00 Central European Time (CET), equivalent to 03:00 Eastern European Time (EET).

Second

Base unit of time in the International System of Units (SI) (Système International d’unités), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each (24 × 60 × 60 = 86400).

A pendulum-governed escapement of a clock, ticking every second

The international standard for timekeeping is Coordinated Universal Time (UTC).