Editing ISO 8601 (section)

==Dates==
{{ISOCALENDAR}}
The standard uses the [[Gregorian calendar]], which "serves as an international standard for civil use".<ref>{{Cite book |last=Doggett |first=L. E. |url=http://astro.nmsu.edu/~lhuber/leaphist.html |title=Explanatory Supplement to the Astronomical Almanac |publisher=University Science Books |year=1992 |isbn=0-935702-68-7 |editor-last=P. K. Seidelmann |location=Sausalito, California |page=580 |chapter=Calendars |quote=The Gregorian calendar today serves as an international standard for civil use. |archive-url=https://web.archive.org/web/20040401234715/http://astro.nmsu.edu/~lhuber/leaphist.html |archive-date=2004-04-01}}</ref>

ISO&nbsp;8601 allows Gregorian dates from the introduction of the calendar on 15 October 1582. For earlier (pre-Gregorian) dates, the calendar may be extended before its introduction (the [[proleptic Gregorian calendar]]) by explicit agreement of the parties involved. Such proleptic dates may not be adjusted to reconcile them with Julian dates.

===Years===
{| class="infobox" style="text-align:center; width:23em"
| YYYY
|-
| ±<u>Y</u>YYYY
|}
ISO&nbsp;8601 prescribes, as a minimum, a four-digit year [YYYY] to avoid the [[year 2000 problem]]. It therefore represents years from 0000 to 9999, year 0000 being equal to 1 [[Anno Domini|BC]] and all others [[AD]], similar to [[astronomical year numbering]]. However, years before 1583 (the first full year following the introduction of the [[Gregorian calendar]]) are not automatically allowed by the standard. Instead, the standard states that "values in the range [0000] through [1582] shall only be used by mutual agreement of the partners in information interchange".<ref>{{Cite book |title=ISO 8601:2004(E) |date=2004-12-01 |publisher=[[International Organization for Standardization|ISO]] |at=section 4.1.2.1 General}}</ref>

To represent years before [[Year zero|0000]] or after [[Year 10,000 problem|9999]], the standard also permits the expansion of the year representation but only by prior agreement between the sender and the receiver.<ref>{{Cite book |title=ISO 8601:2004(E) |date=2004-12-01 |publisher=[[International Organization for Standardization|ISO]] |quote=3.5 Expansion ... By mutual agreement of the partners in information interchange, it is permitted to expand the component identifying the calendar year, which is otherwise limited to four digits. This enables reference to dates and times in calendar years outside the range supported by complete representations, i.e. before the start of the year [0000] or after the end of the year [9999].}}</ref> An expanded year representation [±<u>Y</u>YYYY] must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign<ref>ISO 8601:2004 sections 3.4.2, 4.1.2.4</ref> instead of the more common [[Anno Domini|AD/BC]] (or [[Common Era|CE/BCE]]) notation; by convention 1 BC is labelled [[year zero|+0000]], 2 BC is labeled −0001, and so on.<ref>For example, see Annex B.1.1 of the standard.</ref>

===Calendar dates===
{| class="infobox" style="text-align:center; width:23em"
| {{nowrap|YYYY-MM-DD}} ||''or''|| YYYYMMDD
|-
| {{nowrap|YYYY-MM}}
| colspan="2"| (''but not'' YYYYMM)
|-
| colspan=3 | '''Only allowed in the (now superseded) 2000 version:'''<ref name="datewithoutyear">last in ISO 8601:2000, in use by {{cite IETF
 |rfc=6350|section=4.3.1
 |title=vCard Format Specification |sectionname=DATE
 |access-date=2021-01-21
 |date=August 2011
 |publisher=[[Internet Engineering Task Force|IETF]]
 |quote=Truncated representation, as specified in [ISO.8601.2000], Sections 5.2.1.3 d), e), and f), is permitted.
 |last1=Perreault
 |first1=S.
 }}, although removed in ISO 8601:2004</ref>
|-
| {{nowrap|YY-MM-DD}} ||''or''|| YYMMDD
|-
| {{nowrap|-YY-MM}} ||''or''|| -YYMM
|-
| {{nowrap|--MM-DD}} ||''or''|| --MMDD
|-
| {{nowrap|--MM}}
|-
| {{nowrap|---DD}}
|}

Calendar date representations are in the form shown in the adjacent box. [YYYY] indicates a four-digit year, 0000 through 9999. [MM] indicates a two-digit month of the year, 01 through 12. [DD] indicates a two-digit day of that month, 01 through 31. For example, "5 April 1981" may be represented as either {{nowrap|"1981-04-05"}}<ref name=isofaqtnd/> in the ''extended format'' or "19810405" in the ''basic format''.

The standard also allows for calendar dates to be written with reduced precision. For example, one may write {{nowrap|"1981-04"}} to mean "1981 April". One may simply write "1981" to refer to that year, "198" to refer to the [[decade]] from 1980 to 1989 inclusive, or "19" to refer to the [[century]] from 1900 to 1999 inclusive. Although the standard allows both the {{nowrap|"YYYY-MM-DD"}} and YYYYMMDD formats for complete calendar date representations, if the day [DD] is omitted then only the {{nowrap|YYYY-MM}} format is allowed. By disallowing dates of the form YYYYMM, the standard avoids confusion with the [[#Truncated representations|truncated representation]]<ref name="iso8601amd1" /><ref name="doc2004" /> YYMMDD (still often used). The 2000 version also allowed writing the truncation {{nowrap|"--04-05"}} to mean "April 5"<ref>{{cite IETF
 |rfc=6350|section=4.3.1
 |title=vCard Format Specification |sectionname=DATE
 |access-date=2016-06-29
 |date=August 2011
 |publisher=[[Internet Engineering Task Force|IETF]]
 |quote=Truncated representation, as specified in [ISO.8601.2000], Sections 5.2.1.3 d), e), and f), is permitted.
 |last1=Perreault
 |first1=S.
 }}</ref> but the 2004 version does not allow omitting the year when a month is present.

Examples:

* 7 January 2000 can be written as "2000-01-07" or "20000107"

===Week dates===
{{Main|ISO week date}}
{| class="infobox" style="text-align:center; width:23em"
| YYYY-Www ||''or''|| YYYYWww
|-
| YYYY-Www-D ||''or''|| YYYYWwwD
|}
Week date representations are in the formats as shown in the adjacent box. [YYYY] indicates the ''ISO week-numbering year'' which is slightly different from the traditional [[Gregorian calendar]] year (see below). [Www] is the ''[[week number]]'' prefixed by the letter ''W'', from W01 through W53. [D] is the ''weekday number'', from 1 through 7, beginning with Monday and ending with Sunday.

There are several mutually equivalent and compatible descriptions of week 01:
* the week with the first [[business day]] in the starting year (considering that Saturdays, Sundays and 1 January are non-working days),
* the week with the starting year's first Thursday in it (the formal ISO definition),
* the week with 4 January in it,
* the first week with the majority (four or more) of its days in the starting year, and
* the week starting with the Monday in the period 29 December to 4 January.

As a consequence, if 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year (there is no week 00). 28 December is always in the last week of its year.

The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday of the year.

The ''ISO week-numbering year'' starts at the first day (Monday) of week 01 and ends at the Sunday before the new ISO year (hence without overlap or gap). It consists of 52 or 53 full weeks. The first ISO week of a year may have up to three days that are actually in the Gregorian calendar year that is ending; if three, they are Monday, Tuesday and Wednesday. Similarly, the last ISO week of a year may have up to three days that are actually in the Gregorian calendar year that is starting; if three, they are Friday, Saturday, and Sunday. The Thursday of each ISO week is always in the Gregorian calendar year denoted by the ISO week-numbering year.

Examples:
*{{#time: l j F Y |2008-12-29}} is written "{{#time: o-"W"W-N|2008-12-29}}"
*{{#time: l j F Y |2010-01-03}} is written "{{#time: o-"W"W-N|2010-01-03}}"

===Ordinal dates===
{| class="infobox" style="text-align:center; width:23em"
| YYYY-DDD ||''or''|| YYYYDDD
|}

An ''[[ordinal date]]'' is an [[ordinal number|ordinal]] format for the multiples of a day elapsed since the start of year.
It is represented as "YYYY-DDD" (or YYYYDDD), where [YYYY] indicates a year and [DDD] is the "day of year", from 001 through 365 (366 in [[leap year]]s). For example, {{nowrap|"1981-04-05"}} is the same as {{nowrap|"1981-095"}}.

This simple form is preferable for occasions when the arbitrary nature of week and month definitions are more of an impediment than an aid, for instance, when comparing dates from different calendars. 
This format is used with simple hardware systems that have a need for a date system, but where including full calendar calculation software may be a significant nuisance. This system is sometimes referred to as "Julian Date", but this can cause confusion with the astronomical [[Julian day]], a sequential count of the number of days since day 0 beginning {{nowrap|1 January 4713 BC}} Greenwich noon, Julian proleptic calendar (or noon on ISO date {{nowrap|−4713-11-24}} which uses the Gregorian proleptic calendar with a year 0000).