There's a consistent effort to make all the SI metric units based off of discreet measurements of physical constants. The speed of light is a constant, and the SI 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 9,192,631,770 when expressed in the unit Hz, which is equal to s^–1"