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

World clock showing 12 different time zones
There are 24 time zones across the world (Image credit: Getty Images)

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. That's because everyone sets their local clocks using an internationally agreed standard called Coordinated Universal Time, also known as UTC. 

UTC is defined by an agency of the United Nations called the International Telecommunication Union. It's 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 across the world set their local time by adding or subtracting from UTC depending on their position on the globe.

Laura Mears
Laura Mears

Laura Mears is a keen 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 the first day of the 1960s, shortly after Louis Essen built the first atomic clock. This precision timepiece promised to solve the centuries-old problem of second hands running too fast or too slow. 

Before the 1950s, the most accurate clocks used vibrating quartz crystals to keep time, but the seconds would drift on a daily basis. Essen's invention used the quantum properties of caesium atoms to keep the crystals in sync.

Now, more than 400 extremely stable atomic clocks keep track of time the world over. Each one transmits a signal to the International Bureau of Weights and Measures in France. The Bureau compares them once a month to come up with a final number called the International Atomic Time (TIA). Each clock gets a different weighting in the calculation depending on how stable it is.

Atomic time is so precise that Earth itself can't keep up. In theory, our planet spins on its axis once every 24 hours. But in practice, Earth's rotation is slightly irregular. It fluctuates from day to day, and it's gradually slowing down. 

The irregularity in Earth's spin means that International Atomic Time is now running 37 seconds fast. If we set our clocks by it, we'd soon be waking up for breakfast in the middle of the night.

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

How accurate is the world clock?

To prevent the atomic clocks running away with themselves as Earth slows down, the IERS tries to keep Coordinated Universal Time and Universal Time to 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, some years there have been none at all. In 2020, Earth's rotation actually sped up, making people wonder whether we'd need to remove a leap second for the first time.

The natural oscillations of an atom is more stable than a regular clock pendulum.  (Image credit: Getty images)

The invention of time

Humans have been measuring time for tens of thousands of years. Since the dawn of our species, we have been using Earth's rotation to keep track of the day, first by eye, and then with sundials. Our biggest challenge to begin with was being able to tell time in the dark, especially in the depths 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 didn't appear until the Renaissance. They used weights to move wheels to strike bells to indicate 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, making way for atomic time.

How do atomic clocks work?

Clocks use oscillators to keep time. These devices have periodic behaviour, swinging back and forth in a regular rhythm, 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 a second, generating a wave that rocks up and down in a predictable pattern. The trouble is, it's not completely stable. 

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

To get around this, physicists have locked quartz crystals to the natural resonance of atoms. When exposed to precise frequencies, atoms change their energy state. Detecting these changes makes it possible to monitor the vibration of quartz crystals. So, when quartz clocks drift out of time, we can instantly correct them.

Additional resources

You can learn more about the history of keeping time by watching this TED talk by Karen Mensing. To see the current time in countries all around the world, visit The Clock for Everyone online. 


"Time Zone Map". Time and Date (1995-2022). 

"Ultraprecise atomic optical clocks may redefine the length of a second". Live Science (2022).

"Coordinated Universal Time". Encyclopaedia Britannica (2022). 

"Louis Essen". National Physical Library (2022).

"Bureau International des Poids et Mesures (BIPM)". International Association of Geodesy (2011-2013).

"What is NIST time or UTC(NIST)?". NIST (2022). 

"Earth's spin has slowed. But we still may need a negative leap second.". Live Science (2021).

"First Hourglass". Guinness World Records.

"First-ever mechanical clock". The Seiko Museum Ginza (2022). 

"What Is an Atomic Clock?". NASA (2019).

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Laura Mears

Laura Mears is a biologist who left the confines of the lab for the rigours of an office desk as a keen science writer and a full-time software engineer. Laura has previously written for the magazines How It Works and T3.  Laura's main interests include science, technology and video games.