The time correction D
**UT1**

UT1 (sometimes also written DUT) is the
difference between
**Universal Time**

Universal Time (UT1), which is defined by Earth's
rotation, and Coordinated
**Universal Time**

Universal Time (UTC), which is defined by a
network of precision atomic clocks.

D
**UT1**

UT1 =
**UT1**

UT1 − UTC

**UTC**

UTC is maintained via leap seconds, such that D
**UT1**

UT1 remains within the
range −0.9 s < D
**UT1**

UT1 < +0.9 s. The reason for this correction
is partly that the rate of rotation of the Earth is not constant, due
to tidal braking and the redistribution of mass within the Earth,
including its oceans and atmosphere, and partly because the SI second
(as now used for UTC) was already, when adopted, a little shorter than
the current value of the second of mean solar time.[1]
Forecast values of D
**UT1**

UT1 are published by
**IERS**

IERS Bulletin A.
Weekly updated values of D
**UT1**

UT1 with 0.1 s precision are broadcast
by several time signal services, including WWV and MSF. These services
transmit one pulse per second of some sort. To represent positive DUT1
values from +0.1 to +0.8 seconds, the pulses sent during seconds 1
through 8 are "emphasized" in some way, generally by transmitting a
double pulse. The number of emphasized pulses gives the value of DUT1.
Negative D
**UT1**

UT1 values, from −0.1 to −0.8 seconds, are similarly
represented by emphasizing pulses 9 through 16. For example, a DUT1
value of −0.4 would be transmitted by emphasizing pulses 9 through
12.
The Russian time signal
**RWM** transmits an additional correction d
**UT1**

UT1 in
0.02 s increments. Positive values of d
**UT1**

UT1 from +0.02 to
+0.08 s are encoded by emphasizing pulses 21 through 24; negative
values are encoded by emphasizing pulses 31 through 34. The actual
value of D
**UT1**

UT1 is approximated by the sum of the transmitted D
**UT1**

UT1 +
dUT1.
The longwave RBU time signal also transmits dUT1.
References and notes[edit]

ITU-R Recommendation TF.460-4: Standard-frequency and time-signal
emissions. International Telecommunication Union.

^ :(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C
Hackman and R A Nelson, in Astronomical Journal, vol.136 (2008), pages
1906–1908, it is stated (page 1908), that "the SI second is
equivalent to an older measure of the second of UT1, which was too
small to start with and further, as the duration of the
**UT1**

UT1 second
increases, the discrepancy widens." :(2) In the late 1950s, the
cesium standard was used to measure both the current mean length of
the second of mean solar time (UT2) (result:
7009919263183000000♠9192631830 cycles) and also the second of
ephemeris time (ET) (result: 7009919263177000000♠9192631770±20
cycles), see "
**Time**

Time Scales", by L. Essen, in Metrologia, vol.4 (1968),
pp.161–165, on p.162. As is well known, the
7009919263177000000♠9192631770 figure was chosen for the SI second.
L Essen in the same 1968 article (p.162) stated that this "seemed
reasonable in view of the variations in UT2".

External links[edit]

**IERS**

IERS Rapid Service/Prediction Center (Archived version available for
non-US readers)
http://scienceworld.wolfram.com/astronomy/Time.html
http://maia.usno.navy.mil/ser7/deltat.data

v
t
e

**Time**

Time measurement and standards

Chronometry
Orders of magnitude
Metrology

International standards

Coordinated Universal Time

offset

UT
ΔT
DUT1
International Earth Rotation and Reference Systems Service
ISO 31-1
ISO 8601
International Atomic Time
6-hour clock
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
Solar time
Terrestrial Time
**Time**

Time zone
180th meridian

Obsolete standards

Ephemeris time
Greenwich Mean Time
Prime meridian

**Time**

Time in physics

Absolute time and space
Spacetime
Chronon
Continuous signal
Coordinate time
Cosmological decade
Discrete time and continuous time
Planck time
Proper time
Theory of relativity
**Time**

Time dilation
Gravitational time dilation
**Time**

Time domain
**Time**

Time translation symmetry
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**

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

Chronological dating
Geologic time scale
International Commission on Stratigraphy

Astronomical chronology

Galactic year
Nuclear timescale
Precession
Sidereal time

Other units of time

Flick
Shake
Jiffy
Second
Minute
Moment
Hour
Day
Week
Fortnight
Month
Year
Olympiad
Lustrum
Decade
Century
Saeculum
Millennium

Related topics

Chronology
Duration

music

Mental chronometry
Metric time
System time
**Time**

Time value o