Calendar date
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 September 2026" is ten days after "15 September 2026". 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. The de facto standard for recording dates worldwide is the Gregorian calendar, the world's most widely used civil calendar. Many cultures also use traditional or religious calendars such as those listed in the adjacent table and other lunisolar calendars such as the Buddhist and Hindu calendars.
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. A date without the year may also be referred to as a date or calendar date (such as "14 September" rather than "14 September 2026"). 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 25 March or Christmas on 25 December.