Coordinated Universal Time (UTC)

Say someone in Seoul sets up a video meeting with someone in London. Just saying "three in the afternoon" would make the plan fall apart. You would be calling the same moment by two different numbers.

So the world picked one big clock with a single scale. The time that clock shows is Coordinated Universal Time. Each country's time zone is set by adding hours to this clock or subtracting hours from it.

Korea uses the time that is this clock plus nine hours. That is why schedules write it as UTC+9. A place with no difference from the standard is written as UTC+0.

Computers and servers go further and record time using only this one reference time. They convert it to the local time of the user only when they show it on screen. That way, records that travel between countries do not get their order tangled up.

Which building does this clock hang in? It is not one particular big clock. It is made by averaging the readings of hundreds of atomic clocks kept in labs around the world.

An atomic clock measures time by counting how many times an atom vibrates. It runs so evenly that it would take a very long time to drift by even one second, so these vibrations are now the official standard for the length of one second.

The problem is the Earth. Its spin slows down bit by bit because of the tides, and by uneven amounts. Left alone, the time on the clock and the time of sunrise would slowly drift apart.

So whenever the gap grows, one second is quietly slipped in at the end of a day. That extra tick is called a leap second. It has been added 27 times since 1972, and the last time was on the final day of 2016. However, in 2022 the General Conference on Weights and Measures decided to change the leap second rules by 2035 at the latest, so that in practice no more leap seconds will be added.

You also hear the term GMT a lot. It is the older method that set the time by the movement of the sun along the reference line through Greenwich in London. It is still the label you often see on signs and notices.

If you base time on the sun, the length of a second wobbles every time the Earth's spin wobbles. The current standard instead fixes the second by the vibration of atoms, and fills in only the gap with the sky using leap seconds.

To be more precise, the scales of the two times almost overlap. In daily life you can swap one for the other without trouble, but where split-second timing matters, such as satellite navigation or telecommunications, you need to be clear about which standard you mean.

The name has a backstory too. The English abbreviation and the French abbreviation did not match, so it is said that UTC was chosen as a compromise that follows neither word order.