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Calendar date

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A date in a calendar is a reference to a particular day by means of a calendar system. The calendar date allows the particular day to be identified. A person can often determine how many days a particular date comes after another date. For example, "19 February, 2003", is ten days after "9 February, 2003", in the Gregorian calendar. The most widely used calendar systems are Jesus-based, which is to say they are based on the birthdate of Jesus.

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 in geology.

A date without the year part may also be referred to as a date or calendar date (such as "9 February" rather than "9 February 2003"). As such, it defines the days of an annual event, such as a birthday or Christmas on 25 December.

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Date format

Related to the classification of a day as a specific calendar date is the format used to express that date. The differing formats of dates are an example of endianness.

Even for any specific calendar system, different formats are used. For example, the following formats all express the same date in the Gregorian calendar:

Little endian forms, starting with the day

This sequence is common to the vast majority of the world's countries, and is used as the accepted international date usage.

Big endian forms, starting with the year

This form is consistent with the endianness of the western decimal numbering system, progressing from the highest to the lowest order magnitude.

Middle endian forms, starting with the month

This sequence is common to a smaller number of countries.

This order is used in the United States and countries with U.S. influence (but the U.S. federal government sometimes uses day, month, year). England originally used day, month, year, then for a short while used month, day, year, and finally reverted to the original logical form (day, month, year) which was revived around 1900; the USA chose to stick with month, day, year, but did originally use day, month, year as the English did as can be noted from "the 4th of July" independence day. Canada officially uses the big endian convention, but all three conventions are used in practice.

Usage issues

The many numerical forms can create confusion when used in international correspondence, particularly when abbreviating the year to its final two digits.

When numbers are used to represent months, a significant amount of confusion can arise from the ambiguity of a date order; especially when the numbers representing the day, month or year are low, it can be impossible to tell which order is being used. This can be clarified by using four digits to represent years, and naming the month; for example, "Feb" instead of "02". Many Internet sites use year/month/day, and those using other conventions often write out the month (9-MAY-2001, MAY 09 2001, etc.) so there is no ambiguity. The ISO 8601 date order, with four-digit years, is specifically chosen to be unambiguous.

The ISO 8601 standard also has the advantage of being language independent and therefore is useful when there may be no language context and a universal application is desired (expiration dating on export products, for example). Another advantage is that a plain text list of dates with this format can be easily sorted by word processors, spreadsheets and other software tools with built-in sorting functions.

At least in the United States, dates are rarely written in purely numerical forms in formal writing.

Mixed units, for example feet and inches, or pounds and ounces, are normally written with the largest unit first, in decreasing order. Numbers are also written in that order, so the digits of 2006 indicate, in order, the millennium, the century within the millennium, the decade within the century, and the year within the decade. The only date order that is consistent with these well established conventions is year-month-day.

An early U.S. Federal Information Processing Standard recommended 2-digit years. This is now widely recognized as a bad idea. Even some U.S. government agencies now use ISO 8601 with 4 digit years [link][link].

When transitioning from one date notation to another, people often write both Old Style and New Style dates.

dd/mm/yy (day, month, year) is used by:

Note that '9/11' can refer to both 'The fall of the Berlin Wall' on 9 November 1989 and to the 11 September 2001 attacks on the World Trade Center in the USA.

Note also that in the United Kingdom, while it is regarded as acceptable, but rare, to write monthname, day, year (as well as the little endian day, monthname, year), this order is unacceptable when written numerically.

yyyy-mm-dd (year, month, day), the It is often used in scientific, technical or international communication.

yyyy-mmm-dd

m/d/y (month, day, year) is used by:

Day and year only

The U.S. military sometimes uses a system that indicates the year and day, but not the month. For example, "10 December 1999" can be written in some contexts as "9345", for the 345th day of 1999. This system is most often used on forms.

See also: calendar, time, date-time group, Japanese calendar, [[Wikibooks:English:Time]]

Week number used

Companies in Europe often use year, week number and day, for planning purposes. Since week is a fundamental unit for working life, it makes sense. An event in a project can happen for example w43, w0543 or w543 (week 43 year 2005) or even w43-1 (Monday week 43 year 2005). One problem is that week numbering has different standards, for example the ISO week date, not used by the USA.

See also

External links

 

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