Second

The BIPM's definition: "The second, symbol s, is the SI unit of time. It 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."

Equivalently, as BIPM also puts it: "The second is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom."

Every accuracy claim any watch makes is ultimately a claim measured against this. Adopted by the 26th CGPM in 2018 and published in the SI Brochure, 9th edition, 2019.