And the humble second is no longer 1/86,400th of a day. Due to days not always being 24 hours and the moon stealing our angular momentum and GPS satellites requiring very accurate time, the second is now defined as...
...being equal to the time duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental unperturbed ground-state of the caesium-133 atom.
linkAnd this is almost relevant...