A calendar date is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "25 April 2023" is ten days after "15 April 2023". The date of a particular event depends on the observed time zone. For example, the air attack on Pearl Harbor that began at 7:48 a.m. Hawaiian time on 7 December 1941 took place at 3:18 a.m. Japan Standard Time, 8 December in Japan.

A particular day may be assigned a different nominal date according to the calendar used, so an identifying suffix may be needed where ambiguity may arise. The Gregorian calendar is the world's most widely used civil calendar, and is designated (in English) as AD or CE. Many cultures use religious or regnal calendars such as the Gregorian (Western Christendom, AD), Hebrew calendar (Judaism, AM), the Hijri calendars (Islam, AH), Julian calendar (Eastern Christendom, AD) or any other of the many calendars used around the world. 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 usually counted from a particular starting point, usually 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 "10 April" rather than "10 April 2023"). As such, it is either shorthand for the current year or it defines the day of an annual event, such as a birthday on 31 May, a holiday on 1 September, or Christmas on 25 December.

Many computer systems internally store points in time in Unix time format or some other system time format. The date (Unix) command—internally using the C date and time functions—can be used to convert that internal representation of a point in time to most of the date representations shown here.