The International System of Units (SI) (NIST Special Publication 330, 2008 edition) defines the unit of time, the second as:
"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."
"It follows that the hyperfine splitting in the ground state of the cesium 133 atom is exactly 9 192 631 770 hertz, ν(133Cs)hfs = 9 192 631 770 Hz."
At one point, the second was "considered to be the fraction 1/86 400 of the mean solar day. The exact definition of “mean solar day” was left to the astronomers. However measurements showed that irregularities in the rotation of the Earth made this an unsatisfactory definition."
And by "irregularities" they mean that because earth's rotation is off by a thousandth of a second in some years, this unit is no longer adequate by modernity's exacting standards.
The arbitrary division and subdivision of time (an infinite entity) have been an obsession since the industrial revolution, as scientists continue to divide time into milliseconds, nanoseconds, picoseconds, femtoseconds ...