For the use of date on Wikipedia, see Wikipedia:Manual of Style (dates and numbers)

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, "19 February 2003" is ten days after "9 February 2003" in the Gregorian calendar. The date of a particular event depends on the time zone in which it occurs. For example the attack on Pearl Harbor took place on 7th December 1941, in Hawaii, but on 8th December in Japan.

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 "9 February" rather than "9 February 2003"). As such, it defines the day of an annual event, such as a birthday or Christmas on 25 December.

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 a 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 (see below for breakdown of countries by format).

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.

• 2003 November 16
• 2003-11-16: the ISO 8601 international standard orders the components of a date like this, and additionally uses leading zeros, e.g. 0813-03-01, to be easily read and sorted by computers. It is used with UTC in the Internet date/time format (see the external link below). This format is also favoured in certain Asian countries, mainly East Asian countries, as well as in some Middle and East European countries like Hungary. The big endian convention is also frequently used in Canada, but all three conventions are used there.http://www.cdnpay.ca/news/new_cheque_specs.asp

Middle endian forms, starting with the month

This sequence is used in fewer countries.

• November 16, 2003
• Nov. 16, 2003
• 11/16/2003, 11-16-2003, 11.16.2003 or 11.16.03

Usage issues

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

For example, '9/11' can refer to both 'The fall of the Berlin Wall' on 9 November 1989 and to the September 11 2001 attacks on the World Trade Center in the USA. In the United States, dates are rarely written in purely numerical forms in formal writing. In the United Kingdom, while it is regarded as acceptable, but rare, to write monthname day, year (as well as day monthname year), this order is unacceptable when written numerically, unless referring to the 11 September terrorist attacks - "9/11". ^[1]

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". In some countries Roman numerals are used to denote the month, e.g. 11.IX.2001. 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 is therefore useful when there may be no language context and a universal application is desired (expiration dating on export products, for example).

In addition, the ISO 8601 standard makes sense from a logical perspective. 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. 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.

An early U.S. Federal Information Processing Standard recommended 2-digit years. This is now widely recognized as a bad idea, because of the year 2000 problem. Some U.S. government agencies now use ISO 8601 with 4 digit years [1][2].

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

dd/mm/yyyy (day, month, year)

Using the dd/mm/yyyy format, the 30th of December 2006 would be written as 30/12/2006. The dd/mm/yyyy format is used in:

yyyy-mm-dd (year, month, day) - the ISO 8601 standard

Main article: ISO 8601

Using the yyyy-mm-dd format, the 30th of December 2006 would be written as 2006-12-30. The ISO 8601 standard, yyyy-mm-dd, is used in:



It is often used in scientific, technical or international communication.

mm/dd/yy or mm/dd/yyyy (month, day, year)

Using the mm/dd/yy format, December 30, 2006 would be written as 12/30/06.

The mm/dd/yy format is used in:

• Canada (Although most official documents use the yyyy-mm-dd format, the m/d/yy format is also understood due to influences from the United States.)
• Federated States of Micronesia
• Palau
• Philippines (formerly d/m/yy. May still be found in certain contexts)
• United States (Although Independence Day is often referred to as "the Fourth of July.")

yyyy-mmm-dd (year, month, day)

Using the yyyy-mmm-dd format, the 30th of December 2006 would be written as 2006-Dec-30.

The yyyy-mmm-dd format is used in:

• Canada (as in 2006-JAN-01)

Advantages for Ordering in Sequence

One of the advantages of using the ISO 8601 standard date format is that when dates in this format are ordered sequentially by the leading number, this also orders them in date order, e.g. -

1998-02-28 (28 February 1998) 1999-03-01 (01 March 1999) 2000-01-30 (30 January 2000)

Using the MM-DD-YYYY format, sequential ordering by the leading number would put a list out of date order (as it would be the months being ordered regardless of year):

01-30-2000 (30 January 2000) 02-28-1998 (28 February 1998) 03-01-1999 (01 March 1999)

Using the DD-MM-YYYY format, sequential ordering by the leading number would also put a list out of date order (as it would be the day being ordered regardless of year or month):

01-03-1999 (01 March 1999) 28-02-1998 (28 February 1998) 30-01-2000 (30 January 2000)

Day and year only

See also: , , , , and

The U.S. military sometimes uses a system, which they call "Julian date format"^[2] that indicates the year and the actual day out of the 365 days of the year (and thus a designation of the month would not be needed). For example, "10 December 1999" can be written in some contexts as "1999345" or "99345", for the 345th day of 1999.^[3] This system is most often used in US military logistics, since it makes the process of calculating estimated shipping and arrival dates easier. For example: say a tank engine takes an estimated 35 days to ship by sea from the US to Korea. If the engine is sent on 99104, it should arrive on 99139. Note that outside of the US military, this format is usually referred to as "ordinal date", rather than "Julian date".

Such ordinal date formats are also used by many computer programs (especially those for mainframe systems). Using a three-digit Julian day number saves one byte of computer storage over a two-digit month plus two-digit day, e.g. "January 17" is 017 in Julian versus 0117 in month-day format.

Another "ordinal" date system ("ordinal" in the sense of advancing in value by one as the date advances by one day) is in common use in astronomical calculations and referencing and uses the same name as this "logistics" system. The continuity of representation of period regardless of the time of year being considered is obviously highly useful to both groups of specialists. The astronomers describe their system too as being a "Julian date", and it is described in more detail elsewhere on Wikipedia. Unlike the system described above, the astronomical system does not consider years, it only counts days. Thus it is unperturbed by complications such as leap years.

Week number used

Companies in Europe often use year, week number and day for planning purposes. So, for example, an event in a project can happen on w43 (week 43) or w43-1 (Monday, week 43) or, if the year needs to be indicated, on w0543 or w543 (year 2005 week 43).

The ISO does present a standard for identifying weeks, but as it does not match up with Gregorian calendar (the beginning and ending days of a given year do not match up), this standard is somewhat more problematic than the other standards for dates.

Expressing dates in spoken English

A date written as 7th December (the usual form in the UK) is pronounced "the seventh of December". In the United States and Canada the usual written form is "December 7", pronounced "December (the) seventh".

References

See also

• Date and time notation by country

External links

• IETF: RFC 3339
• W3C Date and Time Formats Internet date/time format
• Date calculator, add or subtract days to or from a given date
• Date and time formats by territory code
• English Dates - explanations, exercises and date generator (written and spoken dates)