Calendar date

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A calendar date is a reference to a particular day, represented within a calendar system, enabling a specific day to be unambiguously identified. Simple math can be performed between dates; commonly, the number of days between two dates may be calculated, e.g., "25 June 2025" is ten days after "15 June 2025". The date of a particular event depends on the time zone used to record it. For example, the air attack on Pearl Harbor that began at 7:48 a.m. local Hawaiian time (HST) on 7 December 1941 is recorded equally as having happened on 8 December at 3:18 a.m. Japan Standard Time (JST).

A particular day may be assigned a different nominal date according to the calendar used.^[a] The de facto standard for recording dates worldwide is the Gregorian calendar, the world's most widely used civil calendar.^[1] Many cultures use religious calendars such as the Gregorian (Western Christendom, AD), the Julian calendar (Eastern Christendom, AD), Hebrew calendar (Judaism, AM), the Hijri calendars (Islam, AH), or any other of the many calendars used around the world. Regnal calendars (that record a date in terms of years since the beginning of the monarch's reign) are also used in some places, for particular purposes.

In most calendar systems, the date consists of three parts: the (numbered) day of the month, the month, and the (numbered) year. There may also be additional parts, such as the day of the week. Years are counted from a particular starting point called the epoch, with era referring to the span of time since that epoch.^[b] A date without the year may also be referred to as a date or calendar date (such as "26 June" rather than "26 June 2025"). As such, it is either shorthand for the current year, or else it defines the day of an annual event such as a birthday on 31 May or Christmas on 25 December.

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