Editing Time standard (section)

== Current time standards ==

[[Geocentric Coordinate Time]] (TCG) is a [[coordinate time]] having its spatial origin at the center of Earth's mass. TCG is a theoretical ideal, and any particular realization will have [[measurement error]].

[[International Atomic Time]] (TAI)<ref>[http://www.bipm.org/en/scientific/tai/tai.html TAI]</ref> is the primary physically realized time standard. TAI is produced by the [[International Bureau of Weights and Measures]] (BIPM), and is based on the combined input of many [[atomic clock]]s around the world,<ref>{{cite web | url=https://www.bipm.org/en/bipm/tai/clock_comparisons.html | archive-url=https://web.archive.org/web/20190810172820/https://www.bipm.org/en/bipm/tai/clock_comparisons.html | archive-date=2019-08-10 | title=BIPM - clock comparisons }}</ref> each corrected for environmental and relativistic effects (both gravitational and because of speed, like in [[Satellite navigation|GNSS]]). TAI is not related to [[Geocentric Coordinate Time|TCG]] directly but rather is a realization of [[Terrestrial Time]] (TT), a theoretical timescale that is a rescaling of TCG such that the time rate approximately matches [[proper time]] at [[mean sea level]].

[[Universal Time]] (UT1) is the [[Earth Rotation Angle]] (ERA) linearly scaled to match historical definitions of [[#Mean solar time|mean solar time]] at 0° longitude. At high precision, Earth's rotation is irregular and is determined from the positions of distant quasars using long baseline interferometry, laser ranging of the Moon and artificial satellites, as well as GPS satellite orbits.

[[Coordinated Universal Time]] (UTC) is an atomic time scale designed to approximate UT1. UTC differs from TAI by an integral number of seconds. UTC is kept within 0.9 second of UT1 by the introduction of one-second steps to UTC, the "[[leap second]]". To date these steps (and difference "TAI-UTC") have always been positive.

The [[Global Positioning System]] broadcasts a very precise [[time signal]] worldwide, along with instructions for converting [[GPS time]] (GPST) to UTC. It was defined with a constant offset from TAI: GPST = TAI - 19 s. The GPS time standard is maintained independently but regularly synchronized with or from, UTC time.

[[Standard time]] or [[civil time]] in a [[time zone]] deviates a fixed, round amount, usually a whole number of hours, from some form of [[Universal Time]], usually UTC.  The offset is chosen such that a new day starts approximately while the Sun is crossing the [[nadir]] meridian. Alternatively the difference is not really fixed, but it changes twice a year by a round amount, usually one hour, see [[Daylight saving time]].

[[Julian day|Julian day number]] is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Conveniently for astronomers, this avoids the date skip during an observation night. Modified Julian day (MJD) is defined as MJD = JD - 2400000.5. An MJD day thus begins at midnight, civil date. Julian dates can be expressed in UT1, TAI, TT, etc. and so for precise applications the timescale should be specified, e.g. MJD 49135.3824 TAI.<ref>{{cite web |last1=Matsakis |first1=Demetrios |title=Systems of time |url=https://tycho.usno.navy.mil/systime.html |archive-url=https://web.archive.org/web/20190930064831/https://tycho.usno.navy.mil/systime.html |access-date=30 September 2019|archive-date=2019-09-30 }}</ref>

[[Barycentric Coordinate Time]] (TCB) is a [[coordinate time]] having its spatial origin at the center of mass of the [[Solar System]], which is called the barycenter.

=== Conversions ===
Conversions between atomic time systems (TAI, GPST, and UTC) are for the most part exact. However, GPS time is a measured value as opposed to a computed "paper" scale.<ref name=Timescales/> As such it may differ from UTC(USNO) by a few hundred nanoseconds,<ref>{{cite web |title=USNO GPS Time Transfer — Naval Oceanography Portal |url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/ |url-status=live |archive-url=https://web.archive.org/web/20220819222419/https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/ |archive-date=2022-08-19 |access-date=2025-01-23 |website=www.cnmoc.usff.navy.mil |quote=GPS time is automatically steered to UTC(USNO) on a daily basis to keep system time within one microsecond of UTC(USNO), but during the last several years has been within a few hundred nanoseconds.}}</ref> which in turn may differ from official UTC by as much as 26 nanoseconds.<ref name="Timescales">{{cite web |title=International Time Scales and the B.I.P.M. — Naval Oceanography Portal |url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/International-Time-Scales-and-the-BIPM/ |archive-url=https://web.archive.org/web/20220819222318/https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/International-Time-Scales-and-the-BIPM/ |archive-date=2022-08-19 |access-date=2025-01-23 |website=www.cnmoc.usff.navy.mil}}</ref> Conversions for UT1 and TT rely on published difference tables which {{as of|2022|lc=y}} are specified to 10 microseconds and 0.1 nanoseconds respectively.
{| class="wikitable"
|-
! System
! Description
! UT1
! UTC
! TT
! TAI
! GPS
|-
| UT1
| Mean Solar Time
| UT1
| UTC = UT1 − DUT1
| TT = UT1 − DUT1 + LS + 32.184 s + DTT
| TAI = UT1 − DUT1 + LS
| GPS = UT1 − DUT1 + LS − 19 s
|-
| UTC
| Civil Time
| UT1 = UTC + DUT1
| UTC
| TT = UTC + LS + 32.184 s + DTT
| TAI = UTC + LS
| GPS = UTC + LS − 19 s
|-
| TT
| Terrestrial Time
| UT1 = TT − 32.184 s − DTT − LS + DUT1
| UTC = TT − 32.184 s − DTT − LS
| TT
| TAI = TT − 32.184 s − DTT
| GPS = TT − 51.184 s − DTT
|-
| TAI
| Atomic Time
| UT1 = TAI − LS + DUT1
| UTC = TAI − LS
| TT = TAI + 32.184 s + DTT
| TAI
| GPS = TAI − 19 s
|-
| GPS
| GPS Time
| UT1 = GPS + 19 s − LS + DUT1
| UTC = GPS + 19 s − LS
| TT = GPS + 51.184 s + DTT
| TAI = GPS + 19 s
| GPS
|}
Definitions:
# LS = TAI − UTC = leap seconds from USNO Table of Leap Seconds<ref>[https://maia.usno.navy.mil/products/leap-second navy.mil]</ref>
# [[DUT1]] = UT1 − UTC published in IERS Bulletins<ref>{{cite web | url=https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html | title=IERS - IERS Bulletins }}</ref> or U.S. Naval Observatory EO<ref>[https://maia.usno.navy.mil/products/eo-products navy.mil]</ref>
# DTT = TT − TAI − 32.184 s published in [[BIPM]]'s TT(BIPM) tables.<ref>{{cite web | url=https://webtai.bipm.org/ftp/pub/tai/ttbipm/ | title=Index of /FTP/Pub/Tai/Ttbipm }}</ref>

TCG is linearly related to TT as: TCG − TT = <var>L<sub>G</sub></var> × (JD − 2443144.5) × 86400 seconds, with the scale difference <var>L<sub>G</sub></var> defined as 6.969290134{{e|-10}} exactly.

TCB is a linear transformation of [[Barycentric Dynamical Time|TDB]] and TDB differs from TT in small, mostly periodic terms. Neglecting these terms (on the order of 2 milliseconds for several millennia around the present epoch),<ref name="IAU2006_B3"/> TCB is related to TT by: TCB − TT = <var>L<sub>B</sub></var> × (JD − 2443144.5) × 86400 seconds.<ref>{{cite web |title=IAU (1991) RECOMMENDATION III |url=https://www.iers.org/IERS/EN/Science/Recommendations/recommendation3.html |website=www.iers.org|at=Note 1}}</ref> The scale difference <var>L<sub>B</sub></var> has been defined by the IAU to be 1.550519768e-08 exactly.<ref name="IAU2006_B3">{{cite web |title=IAU 2006 Resolution B3: Re-definition of Barycentric Dynamical Time, TDB |url=https://www.iau.org/static/resolutions/IAU2006_Resol3.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iau.org/static/resolutions/IAU2006_Resol3.pdf |archive-date=2022-10-09 |url-status=live |access-date=4 April 2022 |page=2}}</ref>