Calendarday

Calendarday is a term used to refer to the unit of civil time that corresponds to a single calendar date. In everyday use, a calendarday runs from midnight to the next midnight in a given time zone and is identified by a year, month, and day label, such as 2025-12-19.

Because it is tied to local civil time, the exact clock length of a calendarday can vary. Daylight saving time transitions can cause a local day to have 23 or 25 hours, though the calendar date progresses as usual.

In computing and data systems, calendarday is often used to mean a date value without a time-of-day component. This abstraction allows applications to represent events, schedules, and milestones anchored to dates rather than times. Programming languages and libraries commonly expose a calendarday-like type or class (for example, a date type that stores year, month, and day) to support calendar-based logic and UI rendering.

The term is sometimes encountered in user interface components, where a calendar grid displays a series of calendardays as selectable cells. It is distinct from related notions such as Julian day numbers, ordinal dates, or timestamps that include a time component.

See also: calendar, date, time zone, ISO 8601, Julian day.