TAI vs UTC vs GPS Time: The Offsets Explained

Three timescales run in parallel behind every accurate clock, and they disagree by whole seconds. The TAI vs UTC distinction trips up engineers regularly, and GPS time adds a third answer that matches neither. None of the three is wrong; each was built for a different purpose, and the offsets between them are fixed, known, and easy to apply once you know which scale you are holding.

This article sets out what each timescale is, the exact offsets that separate them, why satellite time diverges from civil time, and which scale a system ought to use. The three can be read alongside one another on the TAI, UTC and GPS time page.

What Is International Atomic Time?

TAI is the continuous count of SI seconds kept by the world’s atomic clocks. It never pauses, never repeats a second and has no relationship to the Sun. Its initials come from the French, temps atomique international, which is why the letters do not match the English name.

It is not the output of one machine. The BIPM near Paris collects measurements from several hundred clocks in around eighty laboratories, forms a weighted average, and corrects the rate of that average using the handful of primary caesium standards accurate enough to realise the second directly. The result is published monthly rather than broadcast, so no receiver anywhere picks up TAI in real time. What laboratories run day to day are their own realisations, and the published figures tell each of them afterwards how far its clocks stood from the international scale.

TAI vs UTC: How Do They Differ?

UTC is TAI with leap seconds applied. It ticks at exactly the same rate, second for second, but has whole seconds inserted from time to time to keep it aligned with Earth’s rotation, so the two scales differ by an integer that changes on those occasions.

That integer currently stands at 37, following the insertion at the end of 2016. TAI is ahead: subtract 37 seconds and you have UTC. The purpose of the arrangement is to serve two masters at once, giving civil time the uniformity of atomic seconds while keeping midday roughly where the Sun puts it. The constraint is that UTC must stay within 0.9 seconds of UT1, the scale defined by the planet’s rotation angle, and the mechanism for enforcing it is described in leap seconds explained. UTC is what your phone, your operating system and every legal timekeeping system in the world are ultimately set to.

Where GPS Time Fits

GPS time is a third continuous scale, started from UTC at the beginning of 6 January 1980 and never adjusted since. Because it has ignored every leap second inserted in the intervening years, it has steadily pulled ahead of civil time.

The reason is engineering rather than principle. A navigation system solves for position by comparing arrival times of signals to the nanosecond, and a scale that occasionally repeats a second would be a liability in that arithmetic. So the satellites keep an unbroken count, and the navigation message they broadcast includes the current offset from UTC as a separate number. That is how a receiver can display correct civil time while computing with a scale that does not have any. GPS time is steered to stay within a microsecond of the reference kept by the United States Naval Observatory, ignoring leap seconds, and in practice it holds far tighter than that. The onboard clocks that make it possible are covered in atomic clocks in GPS satellites.

TAI vs UTC vs GPS: The Fixed Offsets

Two of the three relationships never change, and the third changes only when a leap second is inserted. As matters stand:

• TAI minus UTC: 37 seconds, unchanged since January 2017 and fixed until another leap second is added, which is now unlikely to happen at all.
• GPS minus UTC: 18 seconds, moving in step with the figure above because both scales are continuous and only UTC leaps.
• TAI minus GPS: exactly 19 seconds, permanently. This offset was fixed when GPS time started in 1980 and can never change, because neither scale takes leaps.
• UTC minus UT1: always less than 0.9 seconds, and continuously varying, since it depends on how fast the Earth happens to be turning.

Other systems made their own choices. GLONASS applies leap seconds and runs on Moscow time, three hours ahead of UTC, while Galileo keeps a continuous scale in the GPS style. A receiver reading several constellations at once has to reconcile all of it, which it does silently.

Which Timescale Should Systems Use?

Use UTC for anything a person or a law will read, and a monotonic clock for measuring how long something took. The mistake that causes real damage is using civil time for durations, because civil time is not guaranteed to move forward smoothly.

Unix time, the counter almost every operating system exposes, follows UTC and inherits its discontinuities: it can repeat a value, and on some systems it can go backwards. Subtracting two such timestamps to measure an interval is therefore unsafe, which is why programming environments provide a separate monotonic clock that only ever increases. Where the length of an interval genuinely matters over long periods, in satellite operations, deep space navigation or physics, systems work in TAI or in an equivalent continuous count and convert to civil time only for display. A short rule covers most cases: store and exchange UTC, timestamp with something monotonic, and treat GPS time as an internal detail of the receiver rather than a scale to hand to anyone. When leap seconds end, the TAI vs UTC offset will simply freeze, and code that follows this rule will need no changes at all.

How the Scales Reach Your Device

Almost everything you own is set from UTC, by one of three routes. The Network Time Protocol carries it over the internet to computers and phones, longwave radio stations broadcast it to clocks in range of a transmitter, and GNSS receivers derive it by applying the broadcast offset to satellite time.

Each route has a different error budget: milliseconds over the internet, tens of milliseconds by radio, and nanoseconds from a well-sited satellite receiver. All three trace back to the same laboratories, described in national time standards. If you want the plain civil figure rather than the machinery behind it, the current UTC clock shows it, and the atomic time display compares it with your own device.

Conclusion

The TAI vs UTC question has a short answer: the same rate, offset by 37 whole seconds. The difference is caused entirely by the leap seconds that atomic time ignores and civil time does not. GPS time is a third continuous scale, 18 seconds ahead of UTC and permanently 19 seconds behind TAI. Use UTC for civil time, a monotonic clock for intervals, and let receivers deal with satellite time internally. To see the three side by side, open the TAI and GPS time comparison, or begin at the atomicclock.now homepage.