## day

https://doi.org/10.1351/goldbook.D01527
Non-SI unit of time, $$\text{d}=86\ 400\ \text{s}$$. The day is not exactly defined in terms of the second since so-called leap seconds are added or subtracted from the day semiannually in order to keep the annual average occurrence of midnight at 24:00 on the clock.
Sources:
Green Book, 2nd ed., p. 111 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 968 [Terms] [Paper]