The ISO WEEK DATE system is effectively a leap week calendar system
that is part of the
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
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 31 December 2006 corresponds to the Sunday of the 52nd week of 2006, and is written 2006-W52-7 (in extended form) or 2006W527 (in compact form). August 2017 WEEK MON TUE WED THU FRI SAT SUN W31 31 01 02 03 04 05 06 W32 07 08 09 10 11 12 13 W33 14 15 16 17 18 19 20 W34 21 22 23 24 25 26 27 W35 28 29 30 31 01 02 03 CONTENTS * 1 Relation with the
* 1.1 First week * 1.2 Last week * 1.3 Weeks per year * 1.4 Weeks per month * 1.5 Dates with fixed week number * 1.6 Equal weeks * 2 Advantages * 3 Disadvantages * 4 Calculation * 4.1 Calculating the week number of a given date * 4.2 Calculating a date given the year, week number and weekday * 5 Other week numbering systems * 6 See also * 7 Notes * 8 External links RELATION WITH THE GREGORIAN CALENDAR The ISO week-numbering year number deviates from the number of the Gregorian year on, if applicable, a Friday, Saturday, and 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, Tuesday and 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 and on all Thursdays the ISO week-numbering year number is always equal to the Gregorian year number. Examples of contemporary dates around New Year’s
GREGORIAN ISO 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 Both years 2007 start with the same day. 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 Gregorian year 2008 is a leap year. ISO year 2008 is 2 days shorter: 1 day longer at the start, 3 days shorter at the end. Sun 28 Dec 2008 2008-12-28 2008-W52-7 Mon 29 Dec 2008 2008-12-29 2009-W01-1 ISO year 2009 begins three days before the end of Gregorian 2008. 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 ISO year 2009 has 53 weeks and ends three days into Gregorian year 2010. 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 FIRST WEEK The
* It is the first week with a majority (4 or more) of its days in January. * Its first day is the Monday nearest to 1 January. * It has 4 January in it. Hence the earliest possible first week extends from Monday 29 December (previous Gregorian year) to Sunday 4 January, the latest possible first week extends from Monday 4 January to Sunday 10 January. * It has the year's first working day in it, if Saturdays, Sundays and 1 January are not working days. If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, it is part of week 53 of the previous year; if on a Saturday, it is part of week 52 (or 53 if the previous Gregorian year was a leap year); if on a Sunday, it is part of week 52 of the previous year. LAST WEEK The last week of the ISO week-numbering year, i.e. the 52nd or 53rd one, is the week before week 01. This week’s properties are: * It has the year's last Thursday in it. * It is the last week with a majority (4 or more) of its days in December. * Its middle day, Thursday, falls in the ending year. * Its last day is the Sunday nearest to 31 December. * It has 28 December in it. 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 (next gregorian year). If 31 December is on a Monday, Tuesday or Wednesday, it is in week 01 of the next year. If it is on a Thursday, it is in week 53 of the year just ending; if on a Friday it is in week 52 (or 53 if the year just ending is a leap year); if on a Saturday or Sunday, it is in week 52 of the year just ending. WEEKS PER YEAR The long years, with 53 weeks in them, can be described by any of the following equivalent definitions: * any year starting on Thursday (dominical letter D or DC ) and any leap year starting on Wednesday (ED) * any year ending on Thursday (D, ED) and any leap year ending on Friday (DC) * years in which 1 January and 31 December (in common years) or either (in leap years) are Thursdays 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 year. Using only the ordinal year number, the number of weeks in that year can be determined: w e e k s ( y e a r ) = 52 + { 1 (long) if p ( y e a r ) = 4 or p ( y e a r 1 ) = 3 0 (short) otherwise p ( y e a r ) = ( y e a r + y e a r 4 y e a r 100 + y e a r 400 ) mod 7 {displaystyle {begin{aligned}weeks(year)&=52+{begin{cases}1{text{ (long)}}&{text{if }}p(year)=4\&{text{or }}p(year-1)=3\0{text{ (short)}}&{text{otherwise}}end{cases}}\p(year) width:63.229ex; height:13.843ex;" alt="{displaystyle {begin{aligned}weeks(year)&=52+{begin{cases}1{text{ (long)}}&{text{if }}p(year)=4\&{text{or }}p(year-1)=3\0{text{ (short)}}&{text{otherwise}}end{cases}}\p(year) years not listed have 52 weeks (364 days); add 2000 for current years: 004, 009, 015, 020, 026, 032, 037, 043, 048, 054, 060, 065, 071, 076, 082, 088, 093, 099, 105, 111, 116, 122, 128, 133, 139, 144, 150, 156, 161, 167, 172, 178, 184, 189, 195, 201, 207, 212, 218, 224, 229, 235, 240, 246, 252, 257, 263, 268, 274, 280, 285, 291, 296, 303, 308, 314, 320, 325, 331, 336, 342, 348, 353, 359, 364, 370, 376, 381, 387, 392, 398. On average, a year has 53 weeks every 400⁄71 = 5.6338… years, and these long ISO years are 43 times 6 years apart, 27 times 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: * 27 Gregorian leap years (366 days, and whose corresponding Julian years are also Julian leap years), emphasized in the list above: * 14 starting on Thursday, hence ending on Friday , and * 13 starting on Wednesday, hence ending on Thursday ; * 44 Gregorian common years (365 days, and whose corresponding Julian years are also Julian common years) starting, hence also ending on Thursday . The Gregorian years corresponding to the other 329 short ISO years (neither starting nor ending with Thursday) can also be subdivided as follows: * 70 are leap Gregorian years (all their corresponding Julian years are also Julian leap years), and * 259 are non-leap Gregorian years (but the corresponding Julian years corresponding to 3 of them are Julian leap years : 100, 200 and 300). Thus, within a 400-year cycle: * 27 week years are 5 days longer than the month years (371 − 366), * 44 week years are 6 days longer than the month years (371 − 365), * 70 week years are 2 days shorter than the month years (364 − 366) and * 259 week years are 1 day shorter than the month years (364 − 365). WEEKS PER MONTH 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
DATES WITH FIXED WEEK NUMBER For all years, 8 days have a fixed ISO week number (between 01 and 08) in January and February. And with the exception of leap years starting on Thursday, dates with fixed week numbers occurs on all months of the year (for 1 day of each ISO week 01 to 52) : Overview of dates with a fixed week number in any year other than a leap year starting on Thursday MONTH DATES WEEK NUMBERS January 04 11 18 25 01–04 February 01 08 15 22 05–08 March 01 08 15 22 29 09–13 April 05 12 19 26 14–17 May 03 10 17 24 31 18–22 June 07 14 21 28 23–26 July 05 12 19 26 27–30 August 02 09 16 23 30 31–35 September 06 13 20 27 36–39 October 04 11 18 25 40–43 November 01 08 15 22 29 44–48 December 06 13 20 27 49–52 During leap years starting on Thursday (i.e. the 13 years number 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 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 Doomsday because for any year, the Doomsday is the day of the week that the last day of February falls on. These dates are one day after the Doomsdays, 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 . EQUAL WEEKS
W10 05 06 07 08 09 10 11 W45 05 06 07 08 09 10 11 W07 12 13 14 15 16 17 18 W11 12 13 14 15 16 17 18 W46 12 13 14 15 16 17 18 W08 19 20 21 22 23 24 25 W12 19 20 21 22 23 24 25 W47 19 20 21 22 23 24 25 The pairs 02/41, 03/42, 04/43, 05/44, 15/28, 16/29, 37/50, 38/51 and triplets 06/10/45, 07/11/46, 08/12/47 have the same days of the month in common years. Of these, the pairs 10/45, 11/46, 12/47, 15/28, 16/29, 37/50 and 38/51 share their days also in leap years. Leap years also have triplets 03/15/28, 04/16/29 and pairs 06/32, 07/33, 08/34. The weeks 09, 19–26, 31 and 35 never share their days of the month with any other week of the same year. ADVANTAGES * All weeks have exactly 7 days, i.e. there are no fractional weeks. * Every week belongs to a single year, i.e. there are no ambiguous or double-assigned weeks. * The date directly tells the weekday. * All week-numbering years start with a Monday and end with a Sunday. * When used by itself without using the concept of month, all week-numbering years are the same except that some years have a week 53 at the end. * The weeks are the same as used with the Gregorian calendar. DISADVANTAGES 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
Solar astronomic phenomena, such as equinox and solstice , vary over a range of at least seven days. This is because each equinox and solstice may occur any day of the week and hence on at least seven different ISO week dates. For example, there are spring equinoxes on 2004-W12-7 and 2010-W11-7. The ISO week calendar relies on the
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 the US, although the work week is usually defined to start on Monday, the week itself is often considered to start on Sunday. CALCULATION CALCULATING THE WEEK NUMBER OF A GIVEN DATE The week number of any date can be calculated, given its ordinal date (i.e. position within the year) and its day of the week . If the ordinal date is not known, it can be computed by any of several methods; perhaps the most direct is a table such as the following. TO THE DAY OF: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ADD: 0 31 59 90 120 151 181 212 243 273 304 334 FOR LEAP YEARS: 0 31 60 91 121 152 182 213 244 274 305 335 Method: Using ISO weekday numbers (running from 1 for Monday to 7 for Sunday), subtract the weekday from the ordinal date, then add 10. Divide the result by 7. Ignore the remainder; the quotient equals the week number. If the week number thus obtained equals 0, it means that the given date belongs to the preceding (week-based) year. If a week number of 53 is obtained, one must check that the date is not actually in week 1 of the following year. w e e k ( d a t e ) = o r d i n a l ( d a t e ) w e e k d a y ( d a t e ) + 10 7 w e e k = { l a s t W e e k ( y e a r 1 ) , w e e k l a s t W e e k ( y e a r ) {displaystyle {begin{aligned}mathrm {week} (mathrm {date} )&=leftlfloor {frac {mathrm {ordinal} (mathrm {date} )-mathrm {weekday} (mathrm {date} )+10}{7}}rightrfloor \mathrm {week} &={begin{cases}mathrm {lastWeek} (mathrm {year} -1),&mathrm {week} ( w e e k d a y ( y e a r ( d a t e ) , 1 , 4 ) + 3 ) i f o r d i n a l d a y s I n Y e a r ( y e a r ) {displaystyle {begin{aligned}mathrm {ordinal(date)} &=mathrm {week(date)times 7+weekday(date)-(weekday(year(date),1,4)+3)} \mathrm {if,ordinal} & Links: ------ /wiki/Leap_week_calendar |