Excel dating system
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 April 2018" is ten days after "14 April 2018" 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 a.m.
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 "9 April" rather than "9 April 2018").
As such, it defines the day of an annual event, such as a birthday or Christmas on 24/25 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.