A date in a calendar 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, "24 June 2012" is ten days after "14 June 2012" in the Gregorian calendar. The date of a particular event depends on the observed UTC offset. For example the attack on Pearl Harbor that began at 1941-12-07T18:18Z took place on 7 December according to Hawaii Time (UTC-10:30), and on 8 December according to Japan Standard Time (UTC+09).

A particular day may be represented by a different date in another calendar as in the Gregorian calendar and the Julian calendar, which have been used simultaneously in different places. In most calendar systems, the date consists of three parts: the day of month, month, and the year. There may also be additional parts, such as the day of week. Years are usually counted from a particular starting point, usually called the epoch, with era referring to the particular period of time (Note the different use of the terms in geology).