July 5, 2026 8 min read

What Is UTC?

UTC, Coordinated Universal Time, is the reference clock of the world. Every time zone is defined as an offset from it: New York is UTC-5 in winter, Tokyo is UTC+9 all year. It is computed from atomic clocks in dozens of laboratories, it never changes for daylight saving time, and it is the time behind every "Z" in a server log or a flight plan.

UTC in one paragraph

Coordinated Universal Time is an international time scale built from atomic clocks and kept within a second of the Earth's rotation. Civil time everywhere is UTC plus or minus a fixed offset, so when it is 12:00 UTC it is 21:00 in Tokyo (UTC+9) and 17:30 in New Delhi (UTC+5:30). UTC itself has no summer time. When New York moves its clocks forward in March, its offset changes from UTC-5 to UTC-4; UTC stays where it was.

That fixed reference is the whole point. A meeting written as "15:00 UTC" means one instant for every reader on the planet, which is why pilots, programmers and weather services use it. Our UTC page shows the current UTC time to the second, next to your local time.

Why "UTC" and not "CUT" or "TUC"

The letters look scrambled because they are a compromise. The system was devised in 1970 by a group of experts within the International Telecommunication Union (ITU). In English the abbreviation would have been CUT; in French, for "temps universel coordonné", it would have been TUC. According to NIST, the ITU wanted one abbreviation for every language and chose UTC so that neither language was favored.

You will also see "UT" in astronomy. Universal Time is the family name; UTC is the coordinated, atomic version, and UT1 (below) is the version tied to the Earth's spin.

Who keeps UTC

No single clock shows UTC. The International Bureau of Weights and Measures (BIPM), based at Sèvres near Paris, computes it after the fact from data sent in by national laboratories. NIST describes the ingredients as a weighted average of more than 300 atomic clocks in more than 60 timing laboratories. The steadiest clocks get the most weight, but no clock can dominate: the maximum weight of any one clock is capped at 2.5 divided by the number of clocks.

Because the calculation needs everyone's data, UTC is a "paper" time scale. The BIPM publishes the results each month in Circular T, listing how far each lab's clock was from UTC at five-day intervals. Meanwhile, each lab runs its own real-time copy, written UTC(k): UTC(NIST) in Colorado, UTC(USNO) at the US Naval Observatory, UTC(NPL) in Britain, UTC(PTB) in Germany. These copies are usually within a few nanoseconds of the final figure, and NIST says UTC(NIST) and UTC(USNO) typically agree within 20 nanoseconds.

The unit underneath all of this is the SI second, defined by the caesium-133 atom: a second is 9,192,631,770 cycles of the radiation from a particular transition in that atom. Commercial caesium clocks, NIST notes, keep time to within about 1/3,000,000 of a second per year. The time on your phone ultimately traces back to these labs, a chain covered in our post on how atomic clocks work.

UTC, TAI and UT1

Three time scales are worth knowing, because UTC sits between the other two.

The Earth is a slightly irregular clock. Its spin varies by milliseconds from day to day, so a purely atomic time scale slowly slips away from the Sun. UTC solves this with leap seconds: when UT1 drifts too far, the International Earth Rotation and Reference Systems Service (IERS) announces an extra second, inserted as 23:59:60 at the end of June or December.

When UTC began its current form in 1972, it was 10 seconds behind TAI. Twenty-seven leap seconds later, the gap is 37 seconds, and it has stayed there since the last leap second at the end of 2016. The IERS confirms this twice a year in Bulletin C. Many systems find leap seconds painful, so in 2022 the General Conference on Weights and Measures (CGPM) decided that the allowed gap between UT1 and UTC will be increased in, or before, 2035. The details are in our post on leap seconds.

One more cousin: GPS time. It counts atomic seconds from January 6, 1980 and ignores leap seconds, so it now runs 18 seconds ahead of UTC. Receivers correct for this automatically using a value broadcast by the satellites.

UTC and GMT

For everyday purposes, 12:00 GMT and 12:00 UTC are the same moment. The difference is in what each term means. Greenwich Mean Time began as mean solar time at the Royal Observatory in Greenwich, an astronomical time that modern science calls UT1. Today "GMT" is mostly the name of a time zone, used by the United Kingdom in winter and by countries such as Ghana and Iceland all year, and that zone is defined from UTC. UTC is the standard; GMT is a zone that happens to sit at offset zero.

The practical catch is London in summer: it switches to British Summer Time, UTC+1, while UTC does not move. The full comparison is in UTC vs GMT, and the GMT time zone page lists who uses it.

Converting UTC to your time

Add the offset if it is positive, subtract it if it is negative. The table shows what 12:00 UTC becomes in common zones, in standard time and in daylight saving time. Zones that observe DST switch on different dates: the United States on the second Sunday in March and the first Sunday in November, the European Union and the UK on the last Sundays of March and October, and southeastern Australia on the first Sundays of October and April.

Each abbreviation has its own page with the live time: EST, PST, CET, IST, JST and AEST. The abbreviations are ambiguous outside their home regions, so for anything that matters, write the UTC offset. To check a specific date, convert 12:00 UTC to several cities in our time zone converter, which applies the DST rules for that day.

A common trap is the date. 22:00 UTC on a Monday is already Tuesday morning in Tokyo, and 02:00 UTC on a Tuesday is still Monday evening in Los Angeles. Convert the date along with the hour.

Where you will see UTC

• Aviation and the military. Flight plans, air traffic control and weather reports use UTC, written with a "Z" for Zulu, the radio word for the letter Z. 1430Z is 14:30 UTC. Our post on military time covers the other letter zones.
• Computers. Servers, databases and log files store UTC and convert to local time only on screen. ISO 8601 timestamps end in "Z" for UTC (2026-07-05T06:31:00Z) or carry an explicit offset (2026-07-05T02:31:00-04:00, the same instant).
• Weather and science. Forecast models, satellite data and earthquake reports are stamped in UTC so data from many countries lines up.
• Spaceflight and radio. Launch windows and shortwave schedules are published in UTC, and NIST's radio stations broadcast it continuously.

For a broader picture of how the offsets fit on the map, read how time zones work. To keep UTC on screen while you work, add it to the world clock next to your own city.

Frequently asked questions