Second

The second is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and lastly to 60 seconds each, for a total of 24 × 60 × 60 = 86,400 seconds per day. The current and formal definition in the International System of Units (SI) is more precise:The 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 9192631770 when expressed in the unit Hz, which is equal to s−1. Read more on Wikipedia