Many people use UTC and GMT as if they were the same thing, and for everyday purposes they almost are. Behind them are two very different ways of keeping time: the rotation of the Earth and the vibrations of atoms. Here is how they fit together.
GMT: time from the sun
Greenwich Mean Time, GMT, was originally the mean solar time at the Royal Observatory in Greenwich, London. It was based on the Earth's rotation: noon was when the average sun crossed the meridian. Because the Earth's orbit is not circular, the real sun is sometimes ahead of the clock and sometimes behind, by up to about 16 minutes over the year, so astronomers average this out to give a steady mean time.
GMT became the world's reference in the 19th century, when ships and railways needed a common time, and the Greenwich meridian was chosen as the zero line for longitude in 1884.
Atomic time: counting vibrations
The rotation of the Earth is not perfectly steady. It slows a little because of tides and shifts because of earthquakes, ice melting and movements inside the planet. In the 1950s scientists built clocks that count the vibrations of atoms, which are far more regular.
Since 1967 the second has been defined as 9,192,631,770 periods of the radiation from a cesium-133 atom. The best atomic clocks today lose less than a second in hundreds of millions of years, and international atomic time, TAI, is made by averaging hundreds of them around the world.
UTC: a compromise
Coordinated Universal Time, UTC, was introduced in 1960 and took its modern form in 1972. It ticks at the steady rate of atomic seconds, but it is kept within a second of the Earth's rotation by occasionally adding a leap second. That makes UTC both precise and in step with the sun.
For almost every practical purpose, UTC and GMT are the same, and time zones are usually defined as offsets from UTC. The United Kingdom uses GMT as its winter time, which equals UTC+0, and British Summer Time, UTC+1, in the summer. GMT is therefore a time zone, while UTC is a time standard.
Leap seconds
Between 1972 and 2016, 27 leap seconds were added, always at the end of June 30 or December 31. On those days the last minute had 61 seconds, shown as 23:59:60. The last one was added on December 31, 2016.
Leap seconds are a headache for computers. Many systems assume that a minute has 60 seconds, and a leap second can cause crashes, duplicate timestamps or time that runs backward. Some large companies avoid the problem by smearing the extra second over many hours.
The plan to stop adding leap seconds
In 2022 the international body that sets the standard for the second voted to stop adding leap seconds by 2035. After that, UTC will be allowed to drift away from the Earth's rotation by a larger amount, probably a minute or more, before any correction is considered, a change that would not be needed for decades.
In recent years the Earth has been spinning slightly faster than before, which would even require a negative leap second, a second removed, and that is something no one has tried.
Other time scales
- UT1 is the time defined directly by the Earth's rotation.
- TAI is the pure atomic time, which is currently 37 seconds ahead of UTC.
- GPS time is also atomic and does not include leap seconds, so it is now 18 seconds ahead of UTC.
- Unix time, used in computers, counts seconds since January 1, 1970 and ignores leap seconds.
Why the difference matters little, and when it does
For ordinary use the difference between UTC and GMT is under a second, so you can treat them as the same. Pilots and the military use the word Zulu for UTC, and servers, logs and software usually store UTC so that records from different places can be compared. Problems arise when someone writes GMT in a schedule in July in London, when local time is actually BST, which is one hour ahead and has led to many missed meetings.
How your devices get the time
Phones and computers do not have atomic clocks. They check the time from servers on the internet using the Network Time Protocol, and these servers in turn get it from atomic clocks or GPS satellites. This keeps a normal device accurate to within a few milliseconds.
The clock on this site shows your device's time, checked against internet time so it stays accurate, and the Unix timestamp tool shows the same moment as a count of seconds.