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 " is ten days after "14 " in the Gregorian calendar. The date of a particular event depends on the time zone in which it is observed. For example the attack on Pearl Harbor took place on December 7, 1941, in Hawaii, but on December 8 according to Japanese 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 " " rather than " "). 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). This date format originates from the custom of writing the date as 'the 16th day of November in the year of our Lord 2003' in religious and legal documents, which at one time were the majority of documents created. The format has shortened as more and more people learned to read and write but the order of the elements has remained constant.

• 16/11/2003, 16-11-2003, 16-11-03 or 16.11.2003 (using dot as separator with this sequence has been defined by DIN 5008)

• [The] 16th [of] November 2003 (The 'of' and 'the' are included in speech, however not in writing.)

• Sunday 16 November 2003

• 16 November 2003

• 16 Nov 2003

• 16/xi/03, 16.xi.03, 16-xi.03, or 16.XI.2003 (using the Roman numeral for the month) - This is usually confined to handwriting only and is not put into any form of print. It is associated with a number of schools and universities. It has also been used by the Vatican as an alternative to using months named after Roman deities.

• 16 November 2003 AD

Big endian forms, starting with the year

In this format the most significant data item is written before lesser data items i.e. year before month before day. This form is standard in Asian countries, Hungary and Sweden. It is consistent with the big endianness of the western decimal numbering system, which progresses from the highest to the lowest order magnitude. That is, using this format alphabet orderings and chronological orderings are identical.

• 2003 November 16

• 2003Nov16

• 2003-Nov-16

• 2003-Nov-16, Sunday

• 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 European countries. The big endian convention is also frequently used in Canada, but all three conventions are used there.

It is also extended through the universal big-endian format clock time: 2003 Nov. 16, 18h 14m 12s, or 2003/11/16/18:14:12 or 2003-11-16T18:14:12.

Middle endian forms, starting with the month

This sequence is used only in the United States and a few other countries (listed below).

• Sunday, November 16, 2003

• 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. 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 never used when written numerically, although, due to American media influence, the American shorthand "9/11" is widely understood as referring to the 11th of September terrorist attacks. (Example of British website using "9/11" shorthand)
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 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 considers its ISO 8601 standard to make 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 year.

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

List of the world locations by date format in use

dd/mm/yyyy, dd.mm.yyyy, or dd-mm-yyyy (day, month, year)

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

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

Using the mm/dd/yy format, October 18, 2008 would be written as 10/18/08.

The mm/dd/yy format is used in:

• Belize

• Canada (Although most official government documents use the yyyy-mm-dd format, the mm/dd/yy format is also understood due to influences from the United States.)

• Federated States of Micronesia

• Palau

• Philippines

• United States (civilian vernacular: mm/dd/yy or mm/dd/yyyy; other formats, including dd Month yyyy and yyyy-mm-dd, are common or prescribed—particularly in military, academic, scientific, computing, industrial, or governmental contexts. See Date and time notation by country#United States.)

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

Using the yyyy-mm-dd format, the 30th of December 2006 would be written as 2006-12-30.

• Used internationally in some contexts as the ISO 8601 standard

Advantages for ordering in sequence

One of the advantages of using the ISO 8601 standard date format is that the lexicographical order of the representations is equivalent to the chronological order of the dates. Thus dates can be sorted using simple string comparison algorithms, and indeed by any left to right collation. For example:

1998-02-28 (28 February 1998) sorts before
1999-03-01 (1 March 1999) which sorts before
2000-01-30 (30 January 2000)

The YYYY MM DD layout is the only format that can provide this. Sorting other date representations involves some parsing of the date strings. This also works when a time in 24-hour format is included after the date, as long as all times are understood to be in the same time zone.

ISO 8601 is used widely where concise, human readable yet easily computable and unambiguous dates are required, although many applications store dates internally as UNIX time and only convert to ISO 8601 for display. It is worth noting that all modern computer Operating Systems retain date information of files outside of their titles, allowing the user to choose which format they prefer and have them sorted thusly, irrespective of the files' names.

Localized date pattern

• Czech Republic d.m.rrrr (den/měsíc/rok)

• France jj/mm/aaaa (jour/mois/année)

• Germany tt.mm.jjjj (Tag/Monat/Jahr)

• Netherlands dd-mm-jjjj (dag/maand/jaar)

• Russia дд/мм/гггг (день/месяц/год)

• Slovakia d.m.rrrr (deň/mesiac/rok)

• Spain dd/mm/aaaa (día/mes/año)

• United States mm/dd/yyyy (month/day/year)

Specialized usage

Day and year only

The U.S. military sometimes uses a system, which they call "Julian date format". 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. 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. OS/390 or its successor, z/OS, display dates in yy.ddd format for most operations. It is important to not confuse any of these formats with UNIX time, which stores time as a number in seconds since 1970-01-01.

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 in the article Julian date. 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

In British English, full dates are usually written and spoken as 7 December 1941 (or 7th December 1941) and pronounced "the seventh of December, nineteen forty-one" (note mandatory usage of "the" and "of"), with the occasional usage of December 7, 1941 ("December the seventh, nineteen forty-one"). In common with continental European usage, however, numerical dates are invariably ordered dd/mm/yyyy.

In the United States, the usual written form is December 7^th, 1941, pronounced "December seventh, nineteen forty-one" or colloquially "December the seventh".

In Australia, Ordinal numerals are not always used when pronouncing dates. For instance, 12/12/09 is sometimes pronounced 12 December 9 and other times 12th December 9.

See also

• Date and time notation by country

• Internationalization and localization