...which is based on seconds, which is based on ??? IDFK (neither would they)
"The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical
value of the caesium frequency, Δν~Cs~, the unperturbed ground-state hyperfine
transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the
unit Hz, which is equal to s−1."
https://www.bipm.org/en/si-base-units/second
"The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, Δν~Cs~, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1." https://www.bipm.org/en/si-base-units/second