TAI
国际原子时 (英语：International Atomic Time、法语：Temps Atomique International, TAI)
1967年第13届国际计量大会上通过一项决议，定义一秒为铯-133原子基态两个超精细能级间跃迁辐射振荡9,192,631,770周所持续的时间。其起点为 世界时 1958年的开始（1958 年 1 月 1 日，TAI-UT1 约为 0）。
UTC
协调世界时（英语：Coordinated Universal Time，法语：Temps Universel Coordonné，简称UTC），与 TAI 相差整数秒。必要时，在 UTC 中引入闰秒，以使 UTC 和 UT 之间的时差小于 0.9 秒。UTC 于 1972 年推出。
UT
世界时（Universal Time，简称UT）是一种以格林威治子夜起算的平太阳时。世界时是以地球自转为基准得到的时间尺度，其精度受到地球自转不均匀变化和极移的影响。以前由天文观测确定，但现在改用 GPS 卫星。 这个时间尺度有点不规则。UT 有多种不同的定义，但它们之间的差异总是小于约 0.03 s。UT1 是目前使用的世界时标准。被作为目前世界民用时间标准UTC在增减闰秒时的参照标准。
TT
地球时（英语：Terrestrial Time），简称TT，是现代在地球表面上的时间标准，是位于大地海平面体验的原时。在天文学，则为局限在地球上的观测者显示星历表的时间座标。它与地心座标时（Geocentric Coordinate Time）有直接的关联，是天文的地球系统时间标准。由于广义相对论中的引力时间膨胀，地球时的滴答声是恒定的比例，并且比地心座标时缓慢些。
Delta-T
delta-T 随地球自转而不断变化。
UT1 相对于 UTC 是可变的。 UTC 中引入了闰秒，以使 delta-UT 保持在 +-0.9 秒之内。
TAI-UTC 始终是整数秒，并且在每年年底、每半年或每三个月添加（或删除，但尚未发生）闰秒时会发生变化。 该优先顺序。
天文时间系统关系
TT = TAI+32.184s ==> UT1-UTC = TAI-UTC - (TT-UT1) + 32.184s
1972 年至今的时间尺度表及一些预测
| Starting at | TAI-UTC | GPS-UTC | TT-UT1 | UT1-UTC |
|---|---|---|---|---|
| 1972-01-01 | +10 | - | +42.23 | -0.05 |
| 1972-07-01 | +11 | - | +42.80 | +0.38 |
| 1973-01-01 | +12 | - | +43.37 | +0.81 |
| 1973-07-01 | " | - | +43.93 | +0.25 |
| 1974-01-01 | +13 | - | +44.49 | +0.69 |
| 1974-07-01 | " | - | +44.99 | +0.19 |
| 1975-01-01 | +14 | - | +45.48 | +0.70 |
| 1975-07-01 | " | - | +45.97 | +0.21 |
| 1976-01-01 | +15 | - | +46.46 | +0.72 |
| 1976-07-01 | " | - | +46.99 | +0.19 |
| 1977-01-01 | +16 | - | +47.52 | +0.66 |
| 1977-07-01 | " | - | +48.03 | +0.15 |
| 1978-01-01 | +17 | - | +48.53 | +0.65 |
| 1978-07-01 | " | - | +49.06 | +0.12 |
| 1979-01-01 | +18 | - | +49.59 | +0.59 |
| 1979-07-01 | " | - | +50.07 | +0.11 |
| 1980-01-01 | +19 | 0 | +50.54 | +0.64 |
| 1980-07-01 | " | " | +50.96 | +0.22 |
| 1981-01-01 | " | " | +51.38 | -0.20 |
| 1981-07-01 | +20 | 1 | +51.78 | +0.40 |
| 1982-01-01 | " | " | +52.17 | +0.01 |
| 1982-07-01 | +21 | 2 | +52.57 | +0.61 |
| 1983-01-01 | " | " | +52.96 | +0.22 |
| 1983-07-01 | +22 | 3 | +53.38 | +0.80 |
| 1984-01-01 | " | " | +53.79 | +0.39 |
| 1984-07-01 | " | " | +54.07 | +0.11 |
| 1985-01-01 | " | " | +54.34 | -0.16 |
| 1985-07-01 | +23 | 4 | +54.61 | +0.57 |
| 1986-01-01 | " | " | +54.87 | +0.31 |
| 1986-07-01 | " | " | +55.10 | +0.08 |
| 1987-01-01 | " | " | +55.32 | -0.14 |
| 1987-07-01 | " | " | +55.57 | -0.39 |
| 1988-01-01 | +24 | 5 | +55.82 | +0.36 |
| 1988-07-01 | " | " | +56.06 | +0.12 |
| 1989-01-01 | " | " | +56.30 | -0.12 |
| 1989-07-01 | " | " | +56.58 | -0.40 |
| 1990-01-01 | +25 | 6 | +56.86 | +0.32 |
| 1990-07-01 | " | " | +57.22 | -0.04 |
| 1991-01-01 | +26 | 7 | +57.57 | +0.61 |
| 1991-07-01 | " | " | +57.94 | +0.24 |
| 1992-01-01 | " | " | +58.31 | -0.13 |
| 1992-07-01 | +27 | 8 | +58.72 | +0.46 |
| 1993-01-01 | " | " | +59.12 | +0.06 |
| 1993-07-01 | +28 | 9 | +59.55 | +0.63 |
| 1994-01-01 | " | " | +59.98 | +0.20 |
| 1994-07-01 | +29 | 10 | +60.38 | +0.80 |
| 1995-01-01 | " | " | +60.78 | +0.40 |
| 1995-07-01 | " | " | +61.20 | -0.02 |
| 1996-01-01 | +30 | 11 | +61.63 | +0.55 |
| 1996-07-01 | " | " | +61.96 | +0.22 |
| 1997-01-01 | " | " | +62.29 | -0.11 |
| 1997-07-01 | +31 | 12 | +62.63 | +0.55 |
| 1998-01-01 | " | " | +62.97 | +0.21 |
| 1998-07-01 | " | " | +63.22 | -0.04 |
| 1999-01-01 | +32 | 13 | +63.47 | +0.71 |
| 1999-07-01 | " | " | +63.66 | +0.52 |
| 2000-01-01 | " | " | +63.82 | +0.36 |
| 2000-07-01 | " | " | +63.98 | +0.20 |
| 2001-01-01 | " | " | +64.09 | +0.09 |
| 2001-07-01 | " | " | +64.20 | -0.02 |
| 2002-01-01 | " | " | +64.30 | -0.12 |
| 2002-07-01 | " | " | +64.41 | -0.23 |
| 2003-01-01 | " | " | +64.47 | -0.29 |
| 2003-07-01 | " | " | +64.55 | -0.37 |
| 2004-01-01 | " | " | +64.57 | -0.39 |
| 2004-07-01 | " | " | +64.65 | -0.47 |
| 2005-01-01 | " | " | +64.68 | -0.50 |
| 2005-07-01 | " | " | +64.80 | -0.62 |
| 2006-01-01 | +33 | 14 | +64.85 | +0.33 |
| 2006-07-01 | " | " | +64.99 | +0.19 |
| 2007-01-01 | " | " | +65.15 | +0.03 |
| 2007-07-01 | " | " | +65.34 | -0.16 |
| 2008-01-01 | " | " | +65.45 | -0.27 |
| 2008-07-01 | " | " | +65.63 | -0.45 |
| 2009-01-01 | +34 | 15 | +65.78 | +0.40 |
| 2009-07-01 | " | " | +65.95 | +0.23 |
| 2010-01-01 | " | " | +66.07 | +0.11 |
| 2010-07-01 | " | " | +66.24 | -0.06 |
| 2011-01-01 | " | " | +66.32 | -0.14 |
| 2011-07-01 | " | " | +66.47 | -0.29 |
| 2012-01-01 | " | " | +66.60 | -0.42 |
| 2012-07-01 | +35 | 16 | +66.77 | +0.41 |
| 2013-01-01 | " | " | +66.91 | +0.27 |
| 2013-07-01 | " | " | +67.13 | +0.05 |
| 2014-01-01 | " | " | +67.28 | -0.10 |
| 2014-07-01 | " | " | +67.49 | -0.31 |
| 2015-01-01 | " | " | +67.64 | -0.46 |
| 2015-07-01 | +36 | 17 | +67.86 | +0.32 |
| 2016-01-01 | " | " | +68.10 | +0.08 |
| 2016-07-01 | " | " | +68.40 | -0.22 |
| 2017-01-01 | +37 | 18 | +68.59 | +0.59 |
| 2017-07-01 | " | " | +68.82 | +0.36 |
| 2018-01-01 | " | " | +68.96 | +0.22 |
| 2018-07-01 | " | " | +69.11 | +0.07 |
| 2019-01-01 | " | " | +69.22 | -0.04 |
| 2019-07-01 | " | " | +69.35 | -0.17 |
| 2020-01-01 | " | " | +69.36 | -0.18 |
| 2020-07-01 | " | " | +69.42 | -0.24 |
| 2021-01-01 | " | " | +69.36 | -0.18 |
| 2021-07-01 | " | " | +69.35 | -0.17 |
| 2022-01-01 | " | " | +69.29 | -0.11 |
| 2022-07-01 | " | " | +69.25 | -0.07 |
| 2023-01-01 | " | " | +69.20 | -0.02 |
| 2023-07-01 | " | " | +69.22 | -0.04 |
| 2024-01-01 | ? | ? | +69 | (pred) |
| 2025-01-01 | ? | ? | +69 | (pred) |
| 2026-01-01 | ? | ? | +69 | (pred) |
| 2027-01-01 | ? | ? | +70 | (pred) |
| 2028-01-01 | ? | ? | +70 | (pred) |
| 2029-01-01 | ? | ? | +70 | (pred) |
Starting at TAI-UTC GPS-UTC TT-UT1 UT1-UTC
(last updated 2023-07-04)