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The ISO week date system is effectively a leap week calendar system that is part of the ISO 8601 date and time standard issued by the International Organization for Standardization (ISO) since 1988 (last revised in 2004) and, before that, it was defined in ISO (R) 2015 since 1971. It is used (mainly) in government and business for fiscal years, as well as in timekeeping. This was previously known as "Industrial date coding". The system specifies a week year atop the Gregorian calendar by defining a notation for ordinal weeks of the year.
The Gregorian leap cycle, which has 97 leap days spread across 400 years, contains a whole number of weeks (20871). In every cycle there are 71 years with an additional 53rd week (corresponding to the Gregorian years that contain 53 Thursdays). An average year is exactly 52.1775 weeks long; months (1⁄12 year) average at exactly 4.348125 weeks.
An ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is sometimes referred to as a leap week, although ISO 8601 does not use this term.
Weeks start with Monday. Each week's year is the Gregorian year in which the Thursday falls. The first week of the year, hence, always contains 4 January. ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.
A precise date is specified by the ISO week-numbering year in the format YYYY, a week number in the format ww prefixed by the letter 'W', and the weekday number, a digit d from 1 through 7, beginning with Monday and ending with Sunday. For example, the Gregorian date Monday 23 December 2019 corresponds to Monday in the 52nd week of 2019, and is written 2019-W52-1 (in extended form) or 2019W521 (in compact form). The ISO year is slightly offset to the Gregorian year; for example, Monday 30 December 2019 in the Gregorian calendar is the first day of week 1 of 2020 in the ISO calendar, and is written as 2020-W01-1 or 2020W011.
|Sat 1 Jan 2005||2005-01-01||2004-W53-6|
|Sun 2 Jan 2005||2005-01-02||2004-W53-7|
|Sat 31 Dec 2005||2005-12-31||2005-W52-6|
|Sun 1 Jan 2006||2006-01-01||2005-W52-7|
|Mon 2 Jan 2006||2006-01-02||2006-W01-1|
|Sun 31 Dec 2006||2006-12-31||2006-W52-7|
|Mon 1 Jan 2007||2007-01-01||2007-W01-1|
|Sun 30 Dec 2007||2007-12-30||2007-W52-7|
|Mon 31 Dec 2007||2007-12-31||2008-W01-1|
|Tue 1 Jan 2008||2008-01-01||2008-W01-2|
|Sun 28 Dec 2008||2008-12-28||2008-W52-7|
|Mon 29 Dec 2008||2008-12-29||2009-W01-1|
|Tue 30 Dec 2008||2008-12-30||2009-W01-2|
|Wed 31 Dec 2008||2008-12-31||2009-W01-3|
|Thu 1 Jan 2009||2009-01-01||2009-W01-4|
|Thu 31 Dec 2009||2009-12-31||2009-W53-4|
|Fri 1 Jan 2010||2010-01-01||2009-W53-5|
|Sat 2 Jan 2010||2010-01-02||2009-W53-6|
|Sun 3 Jan 2010||2010-01-03||2009-W53-7|
The ISO week year number deviates from the Gregorian year number in one of three ways. The days differing are a Friday through Sunday, or a Saturday and Sunday, or just a Sunday, at the start of the Gregorian year (which are at the end of the previous ISO year) and a Monday through Wednesday, or a Monday and Tuesday, or just a Monday, at the end of the Gregorian year (which are in week 01 of the next ISO year). In the period 4 January to 28 December the ISO week year number is always equal to the Gregorian year number. The same is true for every Thursday.
The ISO 8601 definition for week 01 is the week with the first Thursday of the Gregorian year (i.e. of January) in it. The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday:
If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in W01. If it is on a Friday, it is part of W53 of the previous year. If it is on a Saturday, it is part of the last week of the previous year which is numbered W52 in a common year and W53 in a leap year. If it is on a Sunday, it is part of W52 of the previous year.
|Days at the beginning of January||Effect||Days at the end of December|
|W01-1[a]||01 Jan week||…||31 Dec week||1
|G(F)||01||02||03||04||05||06||07||01 Jan||W01||…||W01||31 (30)[c]||(31)|
|F(E)||01||02||03||04||05||06||31 Dec||W01||…||W01||30 (29)||31 (30)||(31)|
|E(D)||01||02||03||04||05||30 Dec||W01||…||W01 (W53)||29 (28)||30 (29)||31 (30)||(31)|
|D(C)||01||02||03||04||29 Dec||W01||…||W53||28 (27)||29 (28)||30 (29)||31 (30)||(31)|
|C(B)||01||02||03||04 Jan||W53||…||W52||27 (26)||28 (27)||29 (28)||30 (29)||31 (30)||(31)|
|B(A)||01||02||03 Jan||W52 (W53)||…||W52||26 (25)||27 (26)||28 (27)||29 (28)||30 (29)||31 (30)||(31)|
|A(G)||01||02 Jan||W52||…||W52 (W01)||25 (31)||26||27||28||29||30||31|
The last week of the ISO week-numbering year, i.e. W52 or W53, is the week before W01 of the next year. This week’s properties are:
Hence the earliest possible last week extends from Monday 22 December to Sunday 28 December, the latest possible last week extends from Monday 28 December to Sunday 3 January.
If 31 December is on a Monday or Tuesday it is in W01 of the next year. If it is on a Wednesday, it is in W01 of the next year in common years and W53 in leap years. If it is on a Thursday, it is in W53 of the year just ending. If on a Friday or Saturday it is in W52 of the year just ending. If on a Sunday, it is in W52 of the year just ending in common years and W01 of the next year in leap years.
|01 Jan||W01-1||Common year (365 − 1 or + 6)||Leap year (366 − 2 or + 5)|
The long years, with 53 weeks in them, can be described by any of the following equivalent definitions:
All other week-numbering years are short years and have 52 weeks.
The number of weeks in a given year is equal to the corresponding week number of 28 December, because it is the only date that is always in the last week of the year since it is a week before 4 January which is always in the first week of the following year.
Using only the ordinal year number y, the number of weeks in that year can be determined:
On average, a year has 53 weeks every 400⁄71 = 5.6338… years, and these long years are 43 × 6 years apart, 27 × 5 years apart, and once 7 years apart (between years 296 and 303). The Gregorian years corresponding to these 71 long years can be subdivided as follows:
The Gregorian years corresponding to the other 329 short years (neither starting nor ending with Thursday) can also be subdivided as follows:
Thus, within a 400-year cycle:
The ISO standard does not define any association of weeks to months. A date is either expressed with a month and day-of-the-month, or with a week and day-of-the-week, never a mix.
Weeks are a prominent entity in accounting where annual statistics benefit from regularity throughout the years. Therefore, in practice usually a fixed length of 13 weeks per quarter is chosen which is then subdivided into 5 + 4 + 4 weeks, 4 + 5 + 4 weeks or 4 + 4 + 5 weeks. The final quarter has 14 weeks in it when there are 53 weeks in the year.
When it is necessary to allocate a week to a single month, the rule for first week of the year might be applied, although ISO 8601 does not consider this case explicitly. The resulting pattern would be irregular. The only 4 months (or 5 in a long year) of 5 weeks would be those with at least 29 days starting on Thursday, those with at least 30 days starting on Wednesday, and those with 31 days starting on Tuesday.
|January||04||11||18||25||W01 – W04|
|February||01||08||15||22||W05 – W08|
|March||01||08||15||22||29||W09 – W13|
|April||05||12||19||26||W14 – W17|
|May||03||10||17||24||31||W18 – W22|
|June||07||14||21||28||W23 – W26|
|July||05||12||19||26||W27 – W30|
|August||02||09||16||23||30||W31 – W35|
|September||06||13||20||27||W36 – W39|
|October||04||11||18||25||W40 – W43|
|November||01||08||15||22||29||W44 – W48|
|December||06||13||20||27||W49 – W52|
For all years, 8 days have a fixed ISO week number (between W01 and W08) in January and February. With the exception of leap years starting on Thursday, dates with fixed week numbers occur in all months of the year (for 1 day of each ISO week W01 to W52).
During leap years starting on Thursday (i.e. the 13 years numbered 004, 032, 060, 088, 128, 156, 184, 224, 252, 280, 320, 348, 376 in a 400-year cycle), the ISO week numbers are incremented by 1 from March to the rest of the year. This last occurred in 1976 and 2004 and will not occur again before 2032. These exceptions are happening between years that are most often 28 years apart, or 40 years apart for 3 pairs of successive years: from year 088 to 128, from year 184 to 224, and from year 280 to 320.
The day of the week for these days are related to the “Doomsday” algorithm, which calculates the weekday that the last day of February falls on. The dates listed in the table are all one day after the Doomsday, except that in January and February of leap years the dates themselves are Doomsdays. In leap years, the week number is the rank number of its Doomsday.
Some pairs and triplets of ISO weeks have the same days of the month:
Some other weeks, i.e. W09, W19 through W26, W31 and W35 never share their days of the month ordinals with any other week of the same year.
Solar astronomic phenomena, such as equinoxes and solstices, vary in the Gregorian calendar over a range spanning three days, over the course of each 400-year cycle, while the ISO Week Date calendar has a range spanning 9 days. For example, there are March equinoxes on 1920-W12-6 and 2077-W11-5 in UT.
The year number of the ISO week very often differs from the Gregorian year number for dates close to 1 January. For example, 29 December 2014 is ISO 2015-W01-1, i.e., it is in year 2015 instead of 2014. A programming bug confusing these two year numbers is probably the cause of some Android users of Twitter being unable to log in around midnight of 29 December 2014 UTC.
The ISO week calendar relies on the Gregorian calendar, which it augments, to define the new year day (Monday of week 01). As a result, extra weeks are spread across the 400-year cycle in a complex, seemingly random pattern. There is no simple algorithm to determine whether a year has 53 weeks from its ordinal number alone. Most calendar reform proposals using leap week designs strive to simplify and harmonize this pattern, some by choosing a different leap cycle (e.g. 293 years).
Not all parts of the world consider the week to begin with Monday. For example, in some Muslim countries, the normal work week begins on Saturday, while in Israel it begins on Sunday. In much of the Americas, although the work week is usually defined to start on Monday, the calendar week is often considered to start on Sunday.
The week number (WW or woy for week of year) of any date can be calculated, given its ordinal date (i.e. day of the year, doy or DDD, 1–365 or 366) and its day of the week (D or dow, 1–7). If the ordinal date is not known, it can be computed from the month (MM or moy) and day of the month (DD or dom) by any of several methods; e.g. using a table such as the following.
The US system has weeks from Sunday through Saturday, and partial weeks at the beginning and the end of the year, i.e. 53 or 54 weeks. An advantage is that no separate year numbering like the ISO year is needed. Correspondence of lexicographical order and chronological order is preserved (just like with the ISO year-week-weekday numbering), but partial weeks make some computations of weekly statistics or payments inaccurate at the end of December or the beginning of January or both.
The US broadcast calendar designates the week containing 1 January (and starting Monday) as the first of the year, but otherwise works like ISO week numbering without partial weeks. Up to six days of the previous December may be part of the first week of the year.
A mix of those, wherein weeks start Sunday and all 1 January is part of the first one, is used in US accounting, resulting in a system with years having also 52 or 53 weeks.