When a website says it shows "atomic time", it is borrowing from one of the quietest achievements in modern science: a worldwide system in which hundreds of clocks agree with each other to a few billionths of a second. Here is how it works.
Until the 1960s, the second was defined by astronomy, as a fraction of the Earth's rotation. But the Earth's spin is slightly irregular, and it is slowing down over long periods. In 1967 the world redefined the second using physics instead. One second is now the duration of 9,192,631,770 cycles of the radiation produced when a caesium-133 atom flips between two particular energy levels.
Every atom of caesium-133 is identical, everywhere, so the definition needs no reference to a pendulum, a crystal or a planet.
A caesium clock does not count atomic vibrations directly. It takes an ordinary electronic oscillator and steers it. Microwaves at a frequency close to 9.19 GHz are directed at a cloud of caesium atoms. If the frequency is exactly right, the largest number of atoms change state. Electronics measure how many did, then nudge the oscillator back toward the perfect frequency. The clock's "ticks" are counted from that locked oscillator.
Modern laboratory standards are called fountain clocks. They toss a ball of laser-cooled atoms upward, and the atoms drift up and fall back through the microwave field. The long interaction time gives an extremely precise reading. National labs report uncertainties on the order of one second in hundreds of millions of years.
Researchers are building clocks that use light frequencies, hundreds of thousands of times higher than microwaves, from atoms such as strontium or ions such as aluminium. They are more precise than caesium, and metrology bodies have been discussing redefining the second around them in the coming years. If that happens, the change would be invisible to you but very important to scientists.
No single clock defines the time you use. The International Bureau of Weights and Measures (BIPM) near Paris collects readings from hundreds of atomic clocks in national laboratories around the world, such as NIST in the US, PTB in Germany, NPL in the UK, and institutes in France, Japan and elsewhere. It combines them into a weighted average called International Atomic Time (TAI), which is extremely stable.
TAI just keeps counting. Because the Earth's rotation is uneven, TAI slowly drifts away from the day–night cycle. So the world uses a second scale, Coordinated Universal Time (UTC), which runs at the same rate as TAI but is occasionally adjusted by whole seconds so it stays within a second of the Earth's rotation.
Those adjustments are leap seconds. Since 1972, 27 of them have been added. The last was at the end of 2016, so UTC is currently 37 seconds behind TAI. Leap seconds have proved a headache for computer systems, because a minute with 61 seconds breaks many programs. In 2022 the international standards community agreed to phase them out by 2035, letting the difference between UTC and Earth rotation grow larger before a correction is made.
Very few people have a caesium clock on a shelf. Instead, atomic time is passed along:
Precise time keeps mobile networks handing over calls, lets banks put trades in order, and stops power grids from falling out of phase. For you it might mean setting a watch, joining a call on time, or checking the exact time in another city. Try our world clocks or the UTC page, which shows the reference time scale itself.
To use that reference in practice, see our guide on how to calibrate a watch.