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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, "24 July 2017" is ten days after "14 July 2017" in the Gregorian calendar. 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 December 7, 1941, took place at 3:18 a.m. December 8 in Japan ( Japan Standard Time ).
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 ).
The most widely used epoch is a conventional birthdate of Jesus (which was established by Dionysius Exiguus in the sixth century). A date without the year part may also be referred to as a date or calendar date (such as "13 July" rather than "13 July 2017"). As such, it defines the day of an annual event, such as a birthday or Christmas on 24/25 December.