What Is TAI? International Atomic Time Defined
The world's most precise time standard runs on roughly 450 atomic clocks in 80 laboratories, weighted and averaged by the Bureau International des Poids et Mesures (BIPM). That average is International Atomic Time. It started in 1958 and has never paused, never jumped, never inserted a leap second.
Every TAI second equals exactly 9,192,631,770 cycles of the radiation emitted by a caesium atom. That definition is fixed. TAI is the pure atomic heartbeat against which every other timescale is measured.
The single most surprising fact about TAI: it is already 37 seconds ahead of the time on your phone, and nobody has announced a plan to stop the gap from growing. Your phone shows UTC. TAI is what UTC would read if we stopped inserting leap seconds.
TAI vs UTC: The Core Difference
UTC equals TAI minus an integer number of seconds. As of 2017, that integer is 37. Right now, UTC is exactly 37 seconds behind TAI.
Why subtract? Because UTC stays within 0.9 seconds of UT1, the astronomical timescale tied to Earth's rotation. Earth's rotation is irregular and gradually slowing. TAI is perfectly uniform. To keep civil time aligned with the Sun, the International Earth Rotation and Reference Systems Service (IERS) periodically inserts leap seconds into UTC. Each one widens the TAI-UTC gap by a second.
The rule of thumb: TAI is the scientist's timescale. UTC is the world's timescale. Run a server, schedule a meeting, catch a flight: use UTC. Timestamp a pulsar observation or a gravitational wave event: use TAI.
Why TAI Has No Leap Seconds
TAI was designed as a continuous coordinate time for physics. Leap seconds would break its primary purpose: providing an uninterrupted, uniform interval for measuring duration.
When a leap second is inserted into UTC, the sequence of seconds goes ... 23:59:59, 23:59:60, 00:00:00. TAI never sees that extra tick. It counts straight through. This is why the International Terrestrial Reference Frame (ITRF), satellite laser ranging, and very long baseline interferometry all rely on TAI. Any break in the count would introduce ambiguity into position measurements measured in millimetres across thousands of kilometres.
The Current TAI-UTC Offset (37 Seconds and Growing)
The current offset of 37 seconds has been in effect since 1 January 2017, when the 27th leap second was added. No further leap seconds have been inserted since that date.
The offset will stay at 37 until the next leap second. Under current rules, the IERS could add another whenever Earth's rotation drifts enough. The General Conference on Weights and Measures (CGPM) voted in 2022 to abolish leap seconds by 2035, which would let the TAI-UTC offset grow indefinitely, potentially to a minute or more. That plan is not yet implemented, and the exact replacement mechanism remains under discussion. Check the IERS Bulletin C for the current status.
Other Timescales: TT, GPS Time, GLONASS Time, Galileo, BeiDou
Each major navigation and scientific system uses its own continuous timescale.
- Terrestrial Time (TT): TAI plus exactly 32.184 seconds. This offset was chosen so that TT matched the former ephemeris time at the epoch 1977 January 1. TT is used for calculating planetary positions and astronomical ephemerides.
- GPS Time: A continuous timescale that started at 00:00:00 UTC on 6 January 1980. GPS time is currently 18 seconds ahead of UTC (because 18 leap seconds have been added to UTC since 1980). GPS time has no leap seconds. GPS satellites carry atomic clocks and broadcast the current UTC offset in their navigation message.
- Galileo System Time (GST): Maintained by the European Union. It is continuous and kept within 50 nanoseconds of UTC (excluding leap seconds). GST is steered toward TAI.
- GLONASS Time (GLONASST): Unlike GPS and Galileo, GLONASS time includes leap seconds. It is set to UTC plus 3 hours (Moscow time) minus a constant offset. GLONASS time synchronises to UTC rather than to TAI.
- BeiDou Time (BDT): The Chinese satellite navigation timescale. BDT is continuous, no leap seconds, and was set to UTC at 00:00:00 UTC on 1 January 2006. It is currently 37 seconds behind TAI (same as UTC).
When TAI Matters: Scientific and Technical Uses
TAI is not a theoretical curiosity. Use it wherever sub-millisecond precision over long periods is non-negotiable.
- Radio telescopes and pulsar timing arrays need TAI because a leap second would corrupt the phase of timing residuals.
- Satellite orbit determination relies on TAI. The ITRF coordinates used by GPS, Galileo, and scientific satellites are defined in a TAI-based frame.
- Network Time Protocol (NTP) servers on the stratum-1 level synchronise to UTC, but the underlying reference is usually TAI from a local atomic clock. The NTP protocol itself uses a 64-bit timestamp counting seconds since 1900; leap-second handling is left to the implementation.
- High-frequency trading firms that timestamp trades for regulatory audit often store both UTC and TAI to eliminate any ambiguity around a leap second.
Quick Reference: TAI vs UTC vs GPS Time
| Timescale | Leap seconds? | Current offset from UTC | Established |
|---|---|---|---|
| TAI | No | UTC + 37 seconds (2017) | 1958 |
| UTC | Yes (27 since 1972) | Reference | 1960 (leap seconds from 1972) |
| GPS Time | No | UTC + 18 seconds | 1980 |
| Terrestrial Time (TT) | No | TAI + 32.184 seconds | 1977 |
| Galileo Time | No | Close to TAI (within 50 ns) | 1999 |
| GLONASS Time | Yes | UTC + 3 hours minus constant | 1982 |
| BeiDou Time | No | UTC + 0 (at epoch) | 2006 |
GPS Time Explained: The 18-Second Offset and No Leap Seconds Since 1980
GPS time is the most widely used continuous timescale after TAI. It started at midnight UTC on 6 January 1980. Since then, 18 leap seconds have been inserted into UTC, so GPS time is now 18 seconds ahead.
GPS receivers calculate local time by subtracting the leap-second offset broadcast in the navigation message. A receiver that cannot decode that message will show GPS time, not UTC. This is why some GPS-based clocks ran exactly 18 seconds fast between 2017 and the present.
GPS time matters for anyone who uses GNSS data in scientific work. Processing raw GPS observations? Convert to TAI or UTC correctly. Using GPS time where UTC is expected will introduce an 18-second error in your timestamps.
Who Should Choose TAI Instead of UTC
The default for almost everyone: use UTC. Your phone, your server, your flight schedule all run on UTC.
Choose TAI if you do any of the following: - Operate a radio telescope or participate in pulsar timing. - Run a stratum-1 NTP server with a local atomic clock. - Work on satellite orbit determination or geodetic VLBI. - Maintain a high-frequency trading system where a leap-second glitch could cost millions.
For everyone else, stick with UTC. It is what the world uses. The 37-second gap between TAI and UTC is real, but it only matters when your work depends on uninterrupted atomic seconds.