World clock: what is it and how does it work?

Have you ever wondered how the World Clock works to keep everyone in sync? We live in different time zones, but from New York to Melbourne, a second is always a second. This is because everyone sets their local clocks using an internationally recognized standard called Coordinated Universal Time, also known as UTC.

UTC is defined by a United Nations agency called International Telecommunication Union (opens in a new tab). It is based on two measurements: the ticking of hundreds of ultra-stable atomic clocks (International Atomic Time) and the rotation of the Earth (Universal Time). Nations around the world set their local time by adding or subtracting UTC based on their position on the globe.

Laura Mears

Laura Mears is an avid science writer and has previously written for our sister publications How It Works and All About Space magazine.

When did the world clock start?

UTC, or the World Clock, has been around since day one in the 1960s, shortly after Louis Essen built the first atomic clock. This precision watch promised to solve the age-old problem of seconds hands spinning too fast or too slow.

Before the 1950s, the most accurate clocks used vibrating quartz crystals to keep time, but the seconds drifted daily. Essen’s invention used the quantum properties of cesium atoms to maintain crystal synchronization.

Today, more than 400 extremely stable atomic clocks keep track of time around the world. Each sends a signal to the International Bureau of Weights and Measures (opens in a new tab) in France. The Bureau compares them once a month to arrive at a final number called the International Atomic Time (TIA) (opens in a new tab). Each clock receives a different weight in the calculation depending on its stability.

Atomic time is so precise that the Earth itself cannot keep up. In theory, our planet rotates on its axis once every 24 hours. But in practice, Earth’s rotation is slightly irregular. It fluctuates from day to day, and it gradually slows down.

The irregularity of the Earth’s rotation means that international atomic time is now running 37 seconds fast. (opens in a new tab)If we set our clocks on it, we would soon wake up for breakfast in the middle of the night.

To account for this natural variation, the world clock also takes into account the Earth’s rotation. The International Earth Rotation and Reference Systems Service (opens in a new tab) (IERS) measure Earth time, known as Universal Time, by observing the stars race past as the planet spins. They then combine this with International Atomic Time to arrive at a final figure for Coordinated Universal Time.

How accurate is the world clock?

To prevent atomic clocks from running away on their own when the Earth slows down, IERS tries to keep Coordinated Universal Time and Universal Time within 0.9 seconds of each other. This involves making regular adjustments called “leap seconds”.

The first leap second was added in 1972, and there have been 26 more since. Some years there have been more than one, others there have been none at all. In 2020, the Earth’s rotation actually accelerated, leading people to wonder if we would need to drop a leap second for the first time.

The natural oscillations of an atom are more stable than an ordinary clock pendulum. (Image credit: Getty images)

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The invention of time

Humans have been measuring time for tens of thousands of years. Since the dawn of our species, we’ve used the Earth’s rotation to track the day, first by eye and then with sundials. Our biggest challenge initially was being able to tell the time in the dark, especially in the dead of winter when the days were short. Solutions included measuring the flow of sand or water, or tracking the length of a burning candle.

The first mechanical clocks did not appear until the Renaissance. They used weights to move wheels to ring bells to tell the hours. Later inventors replaced gravity with springs and spinning wheels with swinging pendulums. Finally, in the 20th century, the first quartz clocks were invented, giving way to atomic time.

How do atomic clocks work?

Clocks use oscillators to keep time. These devices have a periodic behavior, oscillating back and forth at a regular rate, like a pendulum. The faster the swing, the more accurate the clock.

The most common clock oscillator is a quartz crystal. It vibrates thousands of times per second, generating a wave that rises and falls in a predictable pattern. The problem is that it is not completely stable.

Time-sensitive tasks, like moving spacecraft, require clocks that can measure time to billionths of a second. Quartz clocks cannot provide this level of precision.

To circumvent this problem, physicists locked quartz crystals on the natural resonance of atoms (opens in a new tab). When exposed to specific frequencies, atoms change their energy state. Detecting these changes allows the vibration of quartz crystals to be monitored. So when quartz clocks drift out of time, we can correct them instantly.

Additional Resources

You can learn more about the history of time measurement by watching this TED Talk by Karen Mensing (opens in a new tab). To see the current time in countries around the world, visit The clock for everyone (opens in a new tab)on line.


Time zone map (opens in a new tab)“. Time and date (1995-2022).

Ultra-precise atomic optical clocks could redefine how long a second is (opens in a new tab)“. Live Science (2022).

Coordinated Universal Time (opens in a new tab)“. Encyclopedia Britannica (2022).

Louis Essen (opens in a new tab)“. National Library of Physics (2022).

International Bureau of Weights and Measures (BIPM) (opens in a new tab)“. International Association of Geodesy (2011-2013).

What is NIST or UTC(NIST) time? (opens in a new tab)“. NIST (2022).

The rotation of the Earth has slowed down. But we may still need a negative leap second (opens in a new tab).”. Science Live (2021).

First hourglass (opens in a new tab)“. Guinness World Records.

The very first mechanical clock (opens in a new tab)“. The Seiko Ginza Museum (2022).

What is an atomic clock? (opens in a new tab)“. NASA (2019).

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