<p>Remember: If a "second" is 1/86400th of a day, its UT1</p> <p>If a "second" is "defined by taking the fixed numerical value of the caesium frequency, Δ<i>ν</i>_Cs, 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", its UTC/TAI.