second

The second was determined by electromagnetic waves emitted by atoms!?

Symbol: s / Base data: SI base units
The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

It was once based on the length of one day or one year

A second is, roughly speaking, the time it takes for about 9.2 billion emissions of electromagnetic waves by caesium-133 atom. It was once set at one-86,400th of a day and one-31,556,925.9747th of a year, but both fluctuate slightly, so the cesium atomic clock became the standard from 1967.