day

https://doi.org/10.1351/goldbook.D01527
Non-SI unit of time, d = 86400 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 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 968 (https://doi.org/10.1351/pac199668040957)