calendar day

time scale unit [...] starting at the beginning of the day and ending with the beginning of the next day, the latter being the starting instant [...] of the next calendar day [...] Calendar day is in common parlance often referred to as day, however in this document calendar day and day have different definitions. [...] The duration [...] of a calendar day using the 24-hour clock [...] is 24 hours [...]; except if modified by — the insertion or deletion of leap seconds [...], by decision of the IERS, or — the insertion or deletion of other time intervals, as may be prescribed by local authorities to alter the time scale [...] of local time.