What UTC is
Coordinated Universal Time (UTC) is, in the words of NIST, the official, internationally agreed upon standard for world time. It is not the time anywhere in particular; it is the scale the zones are measured from. NIST's own realization, UTC(NIST), is produced from a large group of continuously running atomic clocks whose outputs are combined by weighted averaging, and it agrees with international UTC to within a few nanoseconds. US law uses it directly: 15 U.S.C. 261 divides the country into nine standard time zones, each defined as a fixed number of hours behind or ahead of UTC, and defines UTC as the time scale maintained through the General Conference on Weights and Measures.
Time zones are offsets
A time zone is UTC plus or minus a fixed number of hours, occasionally half or quarter hours. The table shows standard-time offsets for eleven cities and what each one's clock reads at 15:00 UTC on a day in January, when the northern hemisphere is on standard time and Sydney is on daylight time.
| City | Standard offset | Abbreviations | Clock at 15:00 UTC (January) |
|---|---|---|---|
| Honolulu | UTC−10 | HST | 05:00 |
| Los Angeles | UTC−8 | PST / PDT | 07:00 |
| Denver | UTC−7 | MST / MDT | 08:00 |
| Chicago | UTC−6 | CST / CDT | 09:00 |
| New York | UTC−5 | EST / EDT | 10:00 |
| London | UTC+0 | GMT / BST | 15:00 |
| Berlin | UTC+1 | CET / CEST | 16:00 |
| Dubai | UTC+4 | GST | 19:00 |
| Mumbai | UTC+5:30 | IST | 20:30 |
| Tokyo | UTC+9 | JST | 00:00 next day |
| Sydney | UTC+10 | AEST / AEDT | 02:00 next day |
Mumbai's half-hour offset is not an oddity of the table; several countries sit on a half hour, and Nepal on a quarter hour. The abbreviations are local labels, not identifiers: CST means Central Standard Time in Chicago and China Standard Time in Beijing. Software therefore names zones by place, such as America/Chicago, using the IANA Time Zone Database, which also holds every zone's daylight saving rules and their history.
Converting a time
Two steps: subtract the starting zone's offset to reach UTC, then add the destination's offset. A meeting at 09:00 in New York in January, UTC−5, is 14:00 UTC. Add Berlin's +1 for 15:00, Mumbai's +5:30 for 19:30, Tokyo's +9 for 23:00, and Sydney's +11 (daylight time) for 01:00 the next day. The time zone converter does this with the daylight saving rules applied for the date you pick, and the meeting planner lines up several cities to find an hour that is office time for all of them.
Computers avoid the whole problem by storing instants as UTC and converting only for display. A Unix timestamp is the number of seconds since 1 January 1970 00:00 UTC, the same number everywhere on Earth; the Unix timestamp converter turns it into a date in any zone.
Daylight saving time moves the offset
Daylight saving time does not create a new zone; it shifts a zone's offset by one hour for part of the year. In the United States, 15 U.S.C. 260a sets it from 2:00 a.m. on the second Sunday in March to 2:00 a.m. on the first Sunday in November, and NIST lists the places that stay on standard time all year: Hawaii, most of Arizona, American Samoa, Guam, Puerto Rico and the US Virgin Islands. Europe changes on the last Sundays of March and October, Australia's southern states on the first Sundays of October and April, and much of Asia, Africa and South America does not change at all. The result is that the difference between two cities can take two or three values over a year. The daylight saving time guide lists this year's dates by country.
GMT, UT1 and TAI
- GMT, Greenwich Mean Time, is today the name of the UK's winter time zone, UTC+0. In everyday use it is interchangeable with UTC; in precise use UTC is the atomic scale and GMT a zone label.
- UT1 is time measured from the Earth's rotation. The Earth does not rotate perfectly evenly, so UT1 drifts against atomic time.
- TAI, International Atomic Time, is the uninterrupted atomic scale. UTC runs at TAI's rate but is held within 0.9 seconds of UT1 by inserting leap seconds. NIST's table lists 27 of them, the first on June 30, 1972 and the latest on December 31, 2016, which is why UTC is currently 37 seconds behind TAI.
Rules of thumb
- Write times for other people with the zone and, when it matters, the UTC offset: "15:00 CET (UTC+1)" survives a daylight saving change, "3 pm" does not.
- For anything scheduled months ahead across the Atlantic, check the dates around March and October or November, where the usual difference is off by an hour for a few weeks.
- Store and exchange timestamps in UTC; convert at the edge. A countdown to a launch at a fixed UTC instant shows the same remaining time to everyone, whatever their zone.
- Treat zone abbreviations as hints and city names as facts. Two zones can share an abbreviation; no two IANA names do.
Frequently asked questions
What is UTC?
Coordinated Universal Time, the reference clock every time zone is defined against. NIST calls it the official, internationally agreed standard for world time, and US law defines the country's nine time zones as fixed offsets from it. It is kept by atomic clocks and adjusted with leap seconds so it stays within 0.9 seconds of the time given by Earth's rotation.
Is UTC the same as GMT?
For everyday purposes yes: London's winter time is UTC+0, which is what GMT labels. Strictly, GMT is a time zone name and UTC is the atomic time scale the zones are measured from. Timetables, aviation and computing use UTC; "GMT" survives mostly as the name of the UK winter zone and in older documents.
Why do time differences change during the year?
Because daylight saving time moves one place and not the other, or moves them on different dates. New York and London are normally 5 hours apart, but the US changes clocks on the second Sunday in March and the UK on the last Sunday in March, so for two or three weeks each spring and one week each autumn the gap is 4 hours. Sydney changes in the opposite season, so its gap to Europe swings by two hours over the year.
What is a leap second?
One second inserted into UTC to keep it within 0.9 seconds of UT1, the time scale based on Earth's rotation, which runs slightly unevenly. NIST lists 27 of them since the first on June 30, 1972, the most recent on December 31, 2016, so UTC is currently 37 seconds behind International Atomic Time.
How do I convert a time between zones?
Subtract the offset of the place you start from to get UTC, then add the offset of the destination. 9:00 in New York in winter (UTC−5) is 14:00 UTC, which is 15:00 in Berlin (UTC+1) and 23:00 in Tokyo (UTC+9). The time zone converter does this with the daylight saving dates built in, which is where hand conversions go wrong.
Sources
- NIST: Time realization (UTC as the official, internationally agreed standard for world time; UTC(NIST) from a group of atomic clocks, within a few nanoseconds of UTC)
- NIST: Leap seconds (UTC kept within ±0.9 s of UT1; 27 leap seconds from 1972 to 2016; UTC − TAI = 37 s)
- 15 U.S.C. 261: nine US standard time zones defined as offsets from Coordinated Universal Time, UTC as the scale maintained through the General Conference on Weights and Measures
- 15 U.S.C. 260a: US daylight saving time from the second Sunday in March to the first Sunday in November
- NIST: Daylight saving time (the US dates and the places that do not observe it)
- IANA Time Zone Database (the tz database that operating systems and the converter use for zone rules)