The International System of Units then changed things, settling for this definition: "The second… is defined by taking the… ...

at least — is 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a stationary cesium-133 atom at a temperature of 0K.

The International System of Units then changed things, settling for this definition: "The second… is defined by taking the… transition frequency of the caesium-133 atom, to be 9192631770 when ...