Editing Time standard (section)

=== 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>