The second […] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.
Do not matter for relativity though, always same change.
https://en.wikipedia.org/wiki/Second
Do not matter for relativity though, always same change.