Time standards: UTC, TAI, GPS, TT and every other clock of the world
A time standard is an agreed way of counting time. The world runs on UTC, which ticks in atomic seconds from TAI but is kept within 0.9 s of the Earth's rotation (UT1) by leap seconds. Right now TAI = UTC + 37 s, GPS time = UTC + 18 s and Terrestrial Time = UTC + 69.184 s.
Key facts
| World reference | UTC (since 1972) |
|---|---|
| TAI − UTC | 37 seconds (since Jan 1, 2017) |
| GPS − UTC | 18 seconds |
| TT − TAI | 32.184 seconds (fixed) |
| Leap seconds so far | 27 (last on Dec 31, 2016) |
| Leap seconds end | By 2035 |
Every time scale, live
All of these clocks describe the same instant. They differ because each one counts from a different starting point, includes or ignores leap seconds, or follows the Earth instead of atoms. They update from your device clock.
| Time scale | Right now | Compared with UTC |
|---|---|---|
| UTC, Coordinated Universal Time | – | 0 |
| TAI, International Atomic Time | – | +37 s |
| GPS time (Galileo uses the same) | – | +18 s |
| BeiDou time (BDT) | – | +4 s |
| TT, Terrestrial Time | – | +69.184 s |
| GPS week and second of week | – | Counted from Jan 6, 1980 |
| Unix time | – | Seconds since Jan 1, 1970, leap seconds ignored |
| Julian Date | – | Days since noon, Jan 1, 4713 BC |
| Modified Julian Date (MJD) | – | JD − 2,400,000.5 |
| Greenwich sidereal time | – | Earth's rotation against the stars |
| Zulu time (military UTC) | – | 0 |
UT1, the Earth-rotation scale, is not shown live because it can only be measured: it is kept within 0.9 seconds of UTC. TT and sidereal time are shown to the precision a browser can manage.
The three families of time
Every time standard belongs to one of three families, depending on what it counts.
| Family | What it counts | Members |
|---|---|---|
| Atomic time | Vibrations of atoms. Perfectly steady, but has nothing to do with the Sun. | TAI, GPS time, Galileo time, BeiDou time, TT |
| Earth-rotation time | How far the Earth has turned. Matches day and night, but wobbles. | UT1, UT0, UT2, GMT, sidereal time, solar time |
| Civil time | Atomic seconds, nudged now and then to stay close to the Earth. | UTC and every local time zone built on it |
Every major time standard, explained
| Standard | What it is | Who uses it |
|---|---|---|
| UTC Coordinated Universal Time | The world's reference clock. Ticks in atomic seconds, but gets an occasional leap second so it never drifts more than 0.9 s from the Earth's rotation. Every time zone is written as UTC plus or minus an offset. | Everyone: phones, aviation, the internet, science |
| TAI International Atomic Time | The pure atomic count, with no leap seconds. Computed each month by the BIPM near Paris as a weighted average of about 450 atomic clocks in some 80 laboratories. It has been exactly 37 seconds ahead of UTC since January 1, 2017. | Metrology labs; the basis of UTC |
| UT1 Universal Time | Time measured by the Earth's actual rotation, observed with radio telescopes watching distant quasars. The Earth speeds up and slows down slightly, so UT1 drifts against atomic time. | Astronomy, satellite pointing, navigation |
| UT0 / UT2 | UT0 is rotation time as seen from one observatory, before correcting for the wobble of the poles. UT2 also smoothed out seasonal changes; it is now mostly historical. | Historical and specialist use |
| GMT Greenwich Mean Time | Mean solar time at the Greenwich meridian. It was the world standard from 1884 until UTC replaced it in 1972. Today "GMT" is the name of the UK's winter time and is treated as equal to UTC in everyday life. UTC vs GMT. | UK and Ireland in winter, West Africa, Iceland |
| GPS time | The clock of the GPS satellites, started on January 6, 1980 and never adjusted for leap seconds. It is 19 s behind TAI and 18 s ahead of UTC. Receivers subtract the difference using a value broadcast by the satellites. How GPS uses time. | Every GPS receiver, phone networks, power grids |
| Galileo System Time | Europe's satellite navigation clock. Kept aligned with GPS time, so it is also UTC + 18 s. | Galileo receivers |
| BeiDou Time (BDT) | China's satellite navigation clock, started on January 1, 2006 with no leap seconds since. It is 33 s behind TAI and 4 s ahead of UTC. | BeiDou receivers |
| GLONASS time | Russia's satellite navigation clock. Unlike the others it follows UTC as kept in Russia, plus 3 hours, and does take leap seconds. | GLONASS receivers |
| TT Terrestrial Time | The smooth time scale astronomers use for predictions seen from the Earth's surface. Defined as TAI + 32.184 s, so it is 69.184 s ahead of UTC. It replaced Ephemeris Time (ET) in 1984 and keeps continuity with it. | Astronomical almanacs, eclipse predictions |
| TCG and TCB | Relativistic "coordinate" times for the centre of the Earth (TCG) and the centre of mass of the solar system (TCB). Away from the Earth's gravity clocks run faster: TCB gains about half a second a year on TT. | Spacecraft navigation, pulsar timing |
| TDB Barycentric Dynamical Time | A version of solar-system time scaled to stay within about 2 milliseconds of TT. NASA's planetary ephemerides are computed in it. | Solar system ephemerides |
| Unix time | A count of seconds since 00:00 UTC on January 1, 1970, which pretends every day has exactly 86,400 seconds. On a leap second it repeats a value. 32-bit systems overflow on January 19, 2038. Unix timestamp converter. | Computers, databases, the web |
| Julian Date | A running count of days since noon on January 1, 4713 BC, used so astronomers never have to deal with months or calendar reforms. The Modified Julian Date subtracts 2,400,000.5 and starts at midnight. Julian date converter. | Astronomy, satellite data |
| Sidereal time | Time measured by the stars instead of the Sun. A sidereal day is 23 h 56 min 4.09 s, about four minutes shorter than a solar day, because the Earth also moves along its orbit. Sidereal time calculator. | Telescopes, astronomers |
| Apparent solar time | Sundial time: noon is when the Sun is highest. It runs up to about 16 minutes ahead or 14 minutes behind clock time during the year, a difference called the equation of time. | Sundials, solar engineering |
| Mean solar time | Solar time averaged out so every day is the same length. Local mean time differs by 4 minutes for every degree of longitude; time zones replaced it in the late 1800s. | Historically every town; now navigation and astronomy |
| Standard time / civil time / legal time | Standard time is a zone's offset from UTC, such as UTC−5 for US Eastern. Civil time is what the clocks on the wall show, including daylight saving time. Legal time is whatever a country's law says it is. | Everyday life |
Head-to-head comparisons
| Comparison | The difference | Right now |
|---|---|---|
| UTC vs GMT | Same clock reading in daily life. UTC is defined by atomic clocks, GMT by the Sun at Greenwich. | 0 s |
| UTC vs TAI | TAI never takes leap seconds; UTC has taken 27 since 1972. | TAI is 37 s ahead |
| UTC vs UT1 | UT1 follows the real Earth; leap seconds keep it within 0.9 s of UTC. | Under 1 s (measured, not fixed) |
| UTC vs GPS | GPS froze its count in 1980 and skipped every leap second since. | GPS is 18 s ahead |
| UTC vs Unix time | Unix time is UTC written as a number of seconds, ignoring leap seconds. | Same moment, different format |
| Solar time vs standard time | Solar time is set by the Sun overhead; standard time is shared across a whole zone. | Up to an hour or more apart, depending on where you are |
| Sidereal time vs solar time | Sidereal time follows the stars and gains about 3 min 56 s a day on solar time. | One extra sidereal day per year |
Why leap seconds are ending
Leap seconds were added 27 times between 1972 and the last one on December 31, 2016. Each one forces software, banks and networks to handle a minute with 61 seconds, and big outages have followed them. In November 2022 the General Conference on Weights and Measures voted to let UT1 and UTC drift further apart, ending leap seconds by 2035. Lately the Earth has been spinning slightly faster, which raised the possibility of the first ever negative leap second. More in leap seconds explained and the last leap second.
Where the official second comes from
Since 1967 the second has been defined as 9,192,631,770 vibrations of radiation from a cesium-133 atom. Optical clocks using strontium, ytterbium and other atoms are now about 100 times more precise, and metrologists plan to redefine the second around them, possibly from 2030. See how a second is defined, the atomic clock and nuclear clocks.
Sources
- BIPM, Time Department: TAI and UTC, Circular T.
- IERS Bulletin C (leap second announcements) and Bulletin A (UT1−UTC).
- US Naval Observatory, "Systems of Time".
- IS-GPS-200, GPS interface specification (GPS time and week number).
- CGPM 2022, Resolution 4 on the use and future development of UTC.
Questions people ask
What are the main time standards?
The main ones are UTC (the world's civil reference), TAI (pure atomic time), UT1 (Earth-rotation time), GPS time, TT (astronomers' time) and Unix time (computers). All are tied to one another by known offsets.
What is the difference between UTC and TAI?
TAI counts atomic seconds without interruption. UTC uses the same seconds but adds leap seconds to stay close to the Earth's rotation. TAI has been 37 seconds ahead of UTC since January 1, 2017.
Why is GPS time 18 seconds ahead of UTC?
GPS time started in step with UTC on January 6, 1980 and never adds leap seconds. Eighteen leap seconds have been added to UTC since then, so GPS time is 18 seconds ahead.
Is GMT the same as UTC?
For everyday purposes, yes: both read the same time. Technically UTC is defined by atomic clocks and GMT by the mean Sun at Greenwich, and they can differ by up to 0.9 seconds.
What is UT1?
UT1 is time measured by the actual rotation of the Earth. Because the Earth's spin varies slightly, UT1 drifts against atomic time; leap seconds keep UTC within 0.9 seconds of it.
What time standard do computers use?
Most computers keep Unix time, a count of seconds since January 1, 1970 UTC, and synchronise it with UTC over the internet using NTP. They convert it to your local time zone for display.
What is Terrestrial Time?
Terrestrial Time (TT) is the uniform time scale used for astronomical predictions. It equals TAI plus 32.184 seconds, which made it continuous with the older Ephemeris Time.
Last updated October 23, 2026.