Very cool stuff. Warning: the article gets pretty far down into the weeds.

The accuracy of the cesium clock has improved greatly since 1955, increasing by a factor of 10 or so every decade. Nowadays, timekeeping based on cesium clocks accrues errors at a rate of just 0.02 nanosecond per day. If we had started such a clock when Earth began, about 4.5 billion years ago, it would be off by only about 30 seconds today.

But we can do better. A new generation of atomic clocks that use laser light instead of microwave radiation can divide time more finely. About six years ago, researchers completed single-ion versions of these optical clocks, made with an ion of either aluminum or mercury. These surpassed the accuracy of cesium clocks by a full order of magnitude.

Now, a new offshoot of this technology, the optical-lattice clock (OLC), has taken the lead. Unlike single-ion clocks, which yield one measurement of frequency at a time, OLCs can simultaneously measure thousands of atoms held in place by a powerful standing laser beam, driving down statistical uncertainty. In the past year, these clocks have managed to surpass the best single-ion optical clocks in both accuracy and stability. With further development, they will lose no more than a second over 13.8 billion years—the present-day age of the universe.

https://spectrum.ieee.org/aerospace/satellites/introducing-the-worlds-most-precise-clock