. 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. Source:
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996))
on page 968