- Calendar date
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 November 2011" is ten days after "14 November 2011" in the Gregorian calendar. The date of a particular event depends on the observed UTC offset. For example the attack on Pearl Harbor that began at 1941-12-07T18:18Z took place on 7 December according to Hawaii Time (UTC-10:30), and on 8 December according to Japan Standard Time (UTC+09).
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 "21 November" rather than "21 November 2011"). As such, it defines the day of an annual event, such as a birthday or Christmas on 25 December.
- 1 Date format
- 2 World locations by date format in use
- 3 Advantages for ordering in sequence
- 4 Specialized usage
- 5 See also
- 6 References
- 7 External links
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. This date format originates from the custom of writing the date as 'the 9th day of November in the year of our Lord 2003' in western religious and legal documents, which at one time were the majority of documents created. The format has shortened over time but the order of the elements has remained constant.
- "9 November 2003" or "9. November 2003" (the latter is common in German-speaking regions)
- 9/11/2003 or 9.11.2003
- 09-Nov-2003 (used in the USA)
- 9-11-2003 (used in Ireland and the United Kingdom, and is accepted in South Africa, Australia & New Zealand)
- [The] 9th [of] November 2003 (The 'of' and 'the' may be included in speech; they are omitted in all but the most formal writing.)
- Sunday, 9 November 2003
- 9/xi/03, 9.xi.03, 9-xi.03, or 9.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.
- 9 November AD 2003
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. It is consistent with the big endianness of the Indian decimal numbering system, which progresses from the highest to the lowest order magnitude. That is, using this format textual orderings and chronological orderings are identical. This form is standard in Asian countries, Hungary, Sweden and the US armed forces.
Examples for the 9th of November 2003:
- 2003-11-09: the ISO 8601 international standard orders the components of a date like this, and additionally uses leading zeros, for example, 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.
- 2003 November 9
- 2003-Nov-9, Sunday
- 2003. november 9. – The official format in Hungary, point after year and day, month name with small initial. Following shorter formats also can be used: 2003. nov. 9., 2003. 11. 9., 2003. XI. 9.
- 2003.11.9 using dots and no leading zeros, common in China
It is also extended through the universal big-endian format clock time: 9 November 2003, 18h 14m 12s, or 2003/11/9/18:14:12 or (ISO 8601) 2003-11-09T18:14:12.
Middle endian forms, starting with the month
This sequence is used primarily in the United States, partially in Canada, and a few other countries. This date format was commonly used alongside the small endian form in the United Kingdom until the early 20th Century, and can be found in both defunct and modern print media such as the London Gazette and The Times, respectively. In the UK, it would be verbally expressed as Sunday, November the 9th, whereas in the United States, it is usually Sunday, November 9th, although usage of "the" isn't uncommon.
- Sunday, November 9, 2003
- November 9, 2003
- Nov. 9, 2003 or 11/9/2003
- 11-9-2003, 11.9.2003, 11.09.03, or 11/09/03
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 September 11 attacks. In the United States, dates are rarely written in purely numerical forms in formal writing, although they are very common elsewhere; when numerical forms are used, the month appears first. In the United Kingdom, while it is regarded as acceptable to write month-name day, 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 11 September 2001 terrorist attacks.
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, and so on.) 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 years.
When transitioning from one date notation to another, people often write both Old Style and New Style dates.
World locations by date format in use
Advantages for ordering in sequence
One of the advantages of using the ISO 8601 standard date format is that the lexicographical order (ASCIIbetical) 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 common 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 thus, irrespective of the files' names.
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, "11 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 and some US government agencies, including the Internal Revenue Service, 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, for example,, "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 highly useful to both groups of specialists. The astronomers describe their system as also being a "Julian date" system. 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 as 7 December 1941 (or 7th December 1941) and spoken as "the seventh of December, nineteen forty-one" (exceedingly common 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, all-numeric dates are invariably ordered dd/mm/yyyy.
In the United States, the usual written form is December 7, 1941, spoken as "December seventh, nineteen forty-one" or colloquially "December the seventh, nineteen forty-one". Ordinal numerals, however, are not always used when writing and pronouncing dates, and "December seven, nineteen forty-one" is also an accepted pronunciation of the date written December 7, 1941. A notable exception to this rule is the Fourth of July.
- ^ Canadian Payments Association – Specifications for Imageable Cheques and Other Payment Items
- ^ BBC News – America's Day of Terror" (Example of British website using "9/11" shorthand)
- ^ "Numeric representation of Dates and Time". http://www.iso.org/iso/date_and_time_format. Retrieved 2008-04-27.
- ^ http://www.nyecounty.net/iso8601.html
- ^ International Standard Date and Time Notation
- ^ "FAQ: Date formats". World Wide Web Consortium (W3C). http://www.w3.org/International/questions/qa-date-format. Retrieved 2008-10-21.
- ^ Hynes, John (?). A summary of time formats and standards. Retrieved on 2011-02-09 from http://www.decimaltime.hynes.net/p/dates.html.
- ^ Kuhn, Markus (2004-12-19). A summary of the international standard date and time notation. University of Cambridge Computer Laboratory. Retrieved on 2006-08-01 from http://www.cl.cam.ac.uk/~mgk25/iso-time.html.
- ^ Department of Defense. "Definition of Terms." March 11, 1997. Retrieved October 24, 2011.
- IETF: RFC 3339
- W3C Date and Time Formats Internet date/time format
- "Globalization locale database". IBM. http://www-01.ibm.com/software/globalization/locales/index.jsp. Retrieved 2008-10-13. [dead link]
- "NLS (National Language Support) information page". Microsoft. Archived from the original on March 15, 2008. http://web.archive.org/web/20080315070046/http://www.microsoft.com/globaldev/nlsweb/default.mspx. Retrieved 2008-10-13.
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