International System of Units (SI)
A day, symbol d, is defined as 86,400 seconds. The second is the unit of time in SI units.
A day on the UTC time standard can include a negative or positive leap second, and can therefore have a length of 86,399 or 86,401 seconds.
The International Bureau of Weights and Measures (BIPM) currently defines a second as
… the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom.
This makes the SI-based day last exactly 794,243,384,928,000 of those periods.
Read more about this topic: Day
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