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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 January 2014" is ten days after "14 January 2014" 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 7:48 a.m. Hawaiian time on December 7, 1941 took place at 3:18 a.m. December 8 in Japan (Japan Standard 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 "14 January" rather than "14 January 2014"). 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.
There is a large variety of formats for dates in use, which differ in the order of date components (e.g. 31/05/1999, 05/31/1999, 1999/05/31), component separators (e.g. 31.05.1999 vs. 31/05/1999), whether leading zeros are included (e.g. 31/5/1999 vs. 31/05/1999), whether all four digits of the year are written (e.g., 31.05.1999 vs. 31.05.99), and whether the month is represented numerically or by name (e.g. 31 May 1999 vs. 31.05.1999).
This little-endian 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 8th day of November in the year of our Lord 2003" in western religious and legal documents. The format has shortened over time but the order of the elements has remained constant.
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 Greater China, Iran, Japan, South Korea, Belgium, Norway and Sweden; and some other countries to a limited extent.
Examples for the 9th of November 2003:
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.
This sequence is used primarily in the United States. This date format was commonly used alongside the little-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 United States, it is said as Sunday, November 9, although usage of "the" isn't uncommon (e.g. Sunday, November the 9th, and even November the 9th, Sunday, are also possible and readily understood).
When writing a date, do not use the ordinal (th, st, rd, nd) form of numbers when the day follows the month (July 4 or July 4, 1776); however, the ordinal is used when writing a date when the day precedes the month (4 July).
There are several standards that specify date formats:
Many numerical forms can create confusion when used in international correspondence, particularly when abbreviating the year to its final two digits.
For example, "7/8" could refer to either 7 August or July 8. 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 albeit less common to write month-name day, year, this order is never used when written numerically. However, as an exception, 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". The ISO 8601 date order with four-digit years: YYYY-MM-DD (introduced in ISO 2014), 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). Many Internet sites use YYYY-MM-DD, and those using other conventions often use -MMM- for the month to further clarify and avoid ambiguity (2001-MAY-09, 9-MAY-2001, MAY 09 2001, etc.).
In addition, the International Organization for Standardization 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 file managers, word processors, spreadsheets and other software tools with built-in sorting functions. Some database systems use an eight-digit YYYYMMDD representation to handle date values. Naming folders with YYYY-MM-DD at the beginning allows them to be listed in date order when sorting by name - especially useful for organising photograph libraries.
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.
One of the advantages of using the ISO 8601 date format is that the lexicographical order (ASCIIbetical) of the representations is equivalent to the chronological order of the dates, assuming that all dates are in the same time zone. 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.
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 the UNIX Epoch (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.
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.
In English outside of North America, full dates are 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 (U.S. Independence Day).