ISO week date

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 (​112 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 Sunday 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).

May 2019
Week Mon Tue Wed Thu Fri Sat Sun
W18 29 30 01 02 03 04 05
W19 06 07 08 09 10 11 12
W20 13 14 15 16 17 18 19
W21 20 21 22 23 24 25 26
W22 27 28 29 30 31 01 02

Relation with the Gregorian calendar

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.

Examples of contemporary dates around New Year’s Day
Date Notes
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 ISO 8601 definition for week 01 is the week with the Gregorian year's first Thursday in it. The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday:

  • 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 it is on a Saturday, it is part of the last week of the previous year which is numbered 52 in a common year and 53 in a leap year. If it is 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.

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 in common years and week 53 in leap years; if on a Saturday or Sunday, it is in week 52 of the year just ending.

Year start
on (G/W)
Common year
365 − 1 or + 6
Leap year
366 − 2 or + 5
Mon/01 Jan G +0 −1 GF +0 −2
Tue/31 Dec F +1 −2 FE +1 −3
Wed/30 Dec E +2 −3 ED +2 +3
Thu/29 Dec D +3 +3 DC +3 +2
Fri/04 Jan C −3 +2 CB −3 +1
Sat/03 Jan B −2 +1 BA −2 +0
Sun/02 Jan A −1 +0 AG −1 −1

Weeks per year

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 year. Using only the ordinal year number, the number of weeks in that year can be determined:[1]

The following 71 years, shown with light background, in a 400-year cycle have 53 weeks (371 days); years not listed have 52 weeks (364 days); add 2000 for current years:

004 009 015 020 026 28
032 037 043 048 054 56
060 065 071 076 082 84
088 093 099 96
105 111 116 122 124
128 133 139 144 150 152
156 161 167 172 178 180
184 189 195 192
201 207 212 218 220
224 229 235 240 246 248
252 257 263 268 274 276
280 285 291 296 288
303 308 314 316
320 325 331 336 342 344
348 353 359 364 370 372
376 381 387 392 398 400
5 6 5 6 6 28

On average, a year has 53 weeks every ​40071 = 5.6338… years, and these long ISO 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 ISO years (neither starting nor ending with Thursday) can also be subdivided as follows:

  • 70 are Gregorian leap years.
  • 259 are Gregorian common years.

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).
  • 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 ISO 8601 does not consider this case. 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.

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 occur in 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 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 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

Week triplets
W06 05 06 07 08 09 10 11
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.

Week number Calendar date 01
Week table
Jan & Feb
for leap years
01–04 40–43 Jan Oct A B C D E F G 00 06 12 17 23
01–04 14–17 27–30 Jan Apr Jul G A B C D E F 01 07 12 18 24
36–39 49–52 Sep Dec F G A B C D E 02 08 13 19 24
23–26 Jun E F G A B C D 03 08 14 20 25
05–08 09–13 44–48 Feb Mar Nov D E F G A B C 04 09 15 20 26
05–08 05–09 31–35 Feb 29th/Sun Aug C D E F G A B 04 10 16 21 27
18–22 May B C D E F G A 05 11 16 22 00
Year's first 2-digit mod 4 20
Year's last 2-
digit mod 28

Months in the same row are corresponding months and the dates with the same day letter fall on the same weekday. All the D days are the dates with fixed week number. When leap years start on Thursday, the ISO week numbers are incremented by 1 from March to the rest of the year. For the current century letters in column A are domimical letters and years in row C are leap week years (long years). This table can be used to look up dominical letters (DL), day letters (dl), weekdays (w), week numbers (n), and the ISO week date (WD). Letters both in a century column (A C E G) and year rows (c, y) are dominical letters for years of the century. Letters both in day columns and a month row (d, m) are day letters for days of the month.

  • For 1 October 2032 (CD)
c = 20, y = 32 mod 28 = 4, d = 1, m = Oct;
DL = (20, 04/04) = DC, dl = (1, Oct) = A, D = 4 Oct (40 + 1);
C = Sun, A = Fri, D = Mon (41);
n = 41 - 1 = 40, w = 5;
WD = 2032–W40–5.
  • For 1980–W40–1
c = 19, y = 80 mod 28 = 24, n = 40, w = 1 = Mon;
DL = (19, 24/24) = FE, D = 4 Oct (40);
E = Sun, D = Sat (40), F = Mon 6 (41) = Mon 29 Sep (40);
CD = Monday 29 September 1980.


  • 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.


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.[2]

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 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 calendars are simpler in this regard, although they may choose a different leap cycle.

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 and in most of Latin America, although the work week is usually defined to start on Monday, the week itself is often considered to start on Sunday.


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 13
Add 000 031 059 090 120 151 181 212 243 273 304 334 003
Leap year 000 031 060 091 121 152 182 213 244 274 305 335 002
Algorithm od = 30 (m - 1) + floor (0.6 (m + 1)) - i + d
Year's last 2–digit mod 28 (y) 01 02 03 04 05 06
07 08 09 10 11 12
13 14 15 16 17
18 19 20 21 22 23
24 25 26 27 00
Correction (c)
Year's first 2–digit mod 4 (C)
00 00 01 02 03 - 3 - 2 - 1
01 - 2 - 1 00 01 02 03 - 3
02 03 - 3 - 2 - 1 00 01 02
03 01 02 03 - 3 - 2 - 1 00
Algorithm c = (y + floor ((y - 1)/4) + 5 C - 1) mod 7 - 7 if the result > 3

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.

Friday 26 September 2008

  • Ordinal day: 244 + 26 = 270
  • Weekday: Friday = 5
  • 270 − 5 + 10 = 275
  • 275 ÷ 7 = 39.28…
  • Result: Week 39

The week date can also be given by

ceil ((od + c)/7) = week number
(od + c) mod 7 = weekday number

for od and c look up the table above or use the algorithm to calculate. There are 53 weeks in any year (c = 3) or in leap years (c = 2), otherwise there are 52 weeks in a year.

Ceiling the quotient equals the week number and the remainder is the weekday number (0 = Sunday = 7).

For 26 September 2008

ceil ((244 + 26 + 01)/7) = 39
(244 + 26 + 01) mod 7 = 5
od = 30 (9 - 1) + floor (0.6 (9 + 1)) - 2 + 26 = 244 + 26 = 270
c = (8 + floor ((8 - 1)/4) + 5 x 00 - 1) mod 7 = 1
ceil ((270 + 1)/7) = 39
271 mod 7 = 5

the week date is 2008W395.

Calculating a date given the year, week number and weekday

This method requires that one know the weekday of 4 January of the year in question.[3] Add 3 to the number of this weekday, giving a correction to be used for dates within this year.

Method: Multiply the week number by 7, then add the weekday. From this sum subtract the correction for the year. The result is the ordinal date, which can be converted into a calendar date using the table in the preceding section. If the ordinal date thus obtained is zero or negative, the date belongs to the previous calendar year; if greater than the number of days in the year, to the following year.

Example: year 2008, week 39, Saturday (day 6)

  • Correction for 2008: 5 + 3 = 8
  • (39 × 7) + 6 = 279
  • 279 − 8 = 271
  • Ordinal day 271 of a leap year is day 271 − 244 = 27 September
  • Result: 27 September 2008

The ordinal date (od) can also be given by

7 (week number - 1) + weekday number - c

which can be converted into a calendar date, the day (d) of the month (m), using the table above or by the algorithm below:

m = floor (od/30) + 1
d = od mod 30 - floor (0.6 (m + 1)) + i.

For example, as above: 2008W396

od = 7 (39 - 1) + 6 - 1 = 271
d = 271 - 244 = 27 September
m = floor (271/30) + 1 = 10
d = 271 mod 30 - floor (0.6 (10 + 1)) + 2 = -3

October -3 = September 27 (30 - 3).

Other week numbering systems

For an overview of week numbering systems see week number.

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.

A variant of this US scheme groups the possible 1 to 6 days of December remaining in the last week of the Gregorian year within week 1 in January of the next Gregorian year, to make it a full week, bringing a system with accounting years having also 52 or 53 weeks and only the last 6 days of December may be counted as part of another year than the Gregorian year.

The US broadcast calendar counts the week containing 1 January as the first of the year, but otherwise works like ISO week numbering without partial weeks.

See also


  1. ^ Gent, Robert H. "The Mathematics of the ISO 8601 Calendar".
  2. ^
  3. ^ Either see calculating the day of the week, or use this quick-and-dirty method: Subtract 1965 from the year. To this difference add one-quarter of itself, dropping any fractions. Divide this result by 7, discarding the quotient and keeping the remainder. Add 1 to this remainder, giving the weekday number of 4 January. Do not use for years past 2100.

External links

Before Present

Before Present (BP) years is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred in the past. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date of the age scale, reflecting the origin of practical radiocarbon dating in the 1950s. The abbreviation "BP" has alternatively been interpreted as "Before Physics"; that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.In a convention that is not always observed, many sources restrict the use of BP dates to those produced with radiocarbon dating.


Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.

The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.

The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages (biostratigraphy). The aim of chronostratigraphy is to give a meaningful age date to these fossil assemblage intervals and interfaces.


Circa (from Latin, meaning 'around, about, roughly, approximately') – frequently abbreviated c., ca., or ca and less frequently circ. or cca. – signifies "approximately" in several European languages and as a loanword in English, usually in reference to a date. Circa is widely used in historical writing when the dates of events are not accurately known.

When used in date ranges, circa is applied before each approximate date, while dates without circa immediately preceding them are generally assumed to be known with certainty.


1732–1799: Both years are known precisely.

c. 1732 – 1799: The beginning year is approximate; the end year is known precisely.

1732 – c. 1799: The beginning year is known precisely ; the end year is approximate.

c. 1732 – c. 1799: Both years are approximate.

Era (geology)

A geologic era is a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.

The Hadean, Archean and Proterozoic eons were as a whole formerly called the Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, the Archean and Proterozoic eons have been subdivided into eras of their own.

Geologic eras are further subdivided into geologic periods, although the Archean eras have yet to be subdivided in this way.


Floruit (UK: , US: ), abbreviated fl. (or occasionally flor.), Latin for "he/she flourished", denotes a date or period during which a person was known to have been alive or active. In English, the word may also be used as a noun indicating the time when someone flourished.

Fluorine absorption dating

Fluorine absorption dating is a method used to determine the amount of time an object has been underground.

Fluorine absorption dating can be carried out based on the fact that groundwater contains fluoride ions. Items such as bone that are in the soil will absorb fluoride from the groundwater over time. From the amount of absorbed fluoride in the item, the time that the item has been in the soil can be estimated.

Many instances of this dating method compare the amount of fluorine and uranium in the bones to nitrogen dating to create more accurate estimation of date. Older bones have more fluorine and uranium and less nitrogen. But because decomposition happens at different speeds in different places, it's not possible to compare bones from different sites.

As not all objects absorb fluorine at the same rate, this also undermines the accuracy of such a dating technique. Although this can be compensated for by accommodating for the rate of absorption in calculations, such an accommodation tends to have a rather large margin of error.

In 1953 this test was used to easily identify that the 'Piltdown Man' was forged, almost 50 years after it was originally 'unearthed'.

Geologic Calendar

The Geologic Calendar is a scale in which the geological lifetime of the earth is mapped onto a calendrical year; that is to say, the day one of the earth took place on a geologic January 1 at precisely midnight, and today's date and time is December 31 at midnight. On this calendar, the inferred appearance of the first living single-celled organisms, prokaryotes, occurred on a geologic February 25 around 12:30pm to 1:07pm, dinosaurs first appeared on December 13, the first flower plants on December 22 and the first primates on December 28 at about 9:43pm. The first Anatomically modern humans did not arrive until around 11:48 p.m. on New Year's Eve, and all of human history since the end of the last ice-age occurred in the last 82.2 seconds before midnight of the new year.

Geological period

A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.

These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history.

Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.

The rocks formed during a period belong to a stratigraphic unit called a system.

Holocene calendar

The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant (AD/BC or CE/BCE) numbering scheme, placing its first year near the beginning of the Holocene geological epoch and the Neolithic Revolution, when humans transitioned from a hunter-gatherer lifestyle to agriculture and fixed settlements. The year 2019 in the Holocene calendar is 12019 HE. The HE scheme was first proposed by Cesare Emiliani in 1993 (11993 HE).

Law of superposition

The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. It is a form of relative dating. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence. This is important to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.

Leap week calendar

A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, in order to achieve a perennial calendar. Some, however, such as the ISO week date calendar, are simply conveniences for specific purposes.

The ISO calendar in question is a variation of the Gregorian calendar that is used (mainly) in government and business for fiscal years, as well as in timekeeping. In this system a year (ISO year) has 52 or 53 full weeks (364 or 371 days).

Leap week calendars vary on whether the concept of month is preserved and whether the month (if preserved) has a whole number of weeks. The Pax Calendar and Hanke-Henry Permanent Calendar preserve or modify the Gregorian month structure. The ISO week date and the Weekdate Dating System are examples of leap week calendars that eliminate the month.Most leap week calendars take advantage of the 400-year cycle of the Gregorian calendar, which has exactly 20,871 weeks. With 329 common years of 52 weeks plus 71 leap years of 53 weeks, leap week calendars would synchronize with the Gregorian every 400 years (329 × 52 + 71 × 53 = 20,871).


Limmu was an Assyrian eponym. At the beginning of the reign of an Assyrian king, the limmu, an appointed royal official, would preside over the New Year festival at the capital. Each year a new limmu would be chosen. Although picked by lot, there was most likely a limited group, such as the men of the most prominent families or perhaps members of the city assembly. The Assyrians used the name of the limmu for that year to designate the year on official documents. Lists of limmus have been found accounting for every year between 892 BC and 648 BC.

During the Old Assyrian period, the king himself was never the limmum, as it was called in their language. In the Middle Assyrian and Neo-Assyrian periods, however, the king could take this office.

New Earth Time

New Earth Time (or NET) is an alternative naming system for measuring the time of day. In NET the day is split into 360 NET degrees, each NET degree is split into 60 NET minutes and each NET minute is split into 60 NET seconds. One NET degree is therefore equivalent to four standard minutes, and one standard hour is equivalent to 15 NET degrees.

NET is equivalent to the UTC read from a 24-hour analog clock as the clockwise angle past midnight of the hour hand. For example, noon is 180°0'0" NET and at that time the hour hand is pointing straight down forming a 180° angle when measured from the top, at midnight. A full circle is 360 degrees and one NET day.

Nitrogen dating

Nitrogen dating is a form of relative dating which relies on the reliable breakdown and release of amino acids from bone samples to estimate the age of the object. For human bones, the assumption of about 5% nitrogen in the bone, mostly in the form of collogen, allows fairly consistent dating techniques.Compared to other dating techniques, Nitrogen dating can be unreliable because leaching from bone is dependent on temperature, soil pH, ground water, and the presence of microorganism that digest nitrogen rich elements, like collagen. Some studies compare nitrogen dating results with dating results from methods like fluorine absorption dating to create more accurate estimates. Though some situations, like thin porous bones might more rapidly change the dating created by multiple methods.

Proleptic Gregorian calendar

The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. In countries that adopted the Gregorian calendar later, dates occurring in the interim (between 1582 and the local adoption) are sometimes "Gregorianized" as well. For example, George Washington was born on February 11, 1731 (Old Style), as Great Britain and its possessions were using the Julian calendar with English years starting on March 25 until September 1752. After the switch, that day became February 22, 1732, which is the date commonly given as Washington's birthday.


A stratotype or type section is a geological term that names the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the standard of reference for unlayered rocks is the type locality.

Terminus post quem

Terminus post quem ("limit after which", often abbreviated to TPQ) and terminus ante quem ("limit before which", abbreviated to TAQ) specify the known limits of dating for events. A terminus post quem is the earliest time the event may have happened, and a terminus ante quem is the latest. An event may well have both a terminus post quem and a terminus ante quem, in which case the limits of the possible range of dates are known at both ends, but many events have just one or the other. Similarly, terminus ad quem ("limit to which") is the latest possible date of a non-punctual event (period, era, etc.), while terminus a quo ("limit from which") is the earliest. The concepts are similar to those of upper and lower bounds in mathematics.


A timeline is a display of a list of events in chronological order. It is typically a graphic design showing a long bar labelled with dates paralleling it, and usually contemporaneous events; a Gantt chart is a form of timeline used in project management.

Timelines can use any suitable scale representing time, suiting the subject and data; many use a linear scale, in which a unit of distance is equal to a set amount of time. This timescale is dependent on the events in the timeline. A timeline of evolution can be over millions of years, whereas a timeline for the day of the September 11 attacks can take place over minutes, and that of an explosion over milliseconds. While many timelines use a linear timescale -- especially where very large or small timespans are relevant -- logarithmic timelines entail a logarithmic scale of time; some "hurry up and wait" chronologies are depicted with zoom lens metaphors.


A week is a time unit equal to seven days. It is the standard time period used for cycles of rest days in most parts of the world, mostly alongside—although not strictly part of—the Gregorian calendar.

In many languages, the days of the week are named after classical planets or gods of a pantheon. In English, the names are Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday.

ISO 8601 includes the ISO week date system, a numbering system for weeks within a given year – each week begins on a Monday and is associated with the year that contains that week's Thursday (so that if a year starts in a long weekend Friday–Sunday, week number one of the year will start after that). ISO 8601 assigns numbers to the days of the week, running from 1 to 7 for Monday through to Sunday.

The term "week" is sometimes expanded to refer to other time units comprising a few days, such as the nundinal cycle of the ancient Roman calendar, the "work week", or "school week" referring only to the days spent on those activities.

International standards
Obsolete standards
Time in physics
Archaeology and geology
Astronomical chronology
Other units of time
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Astronomic time
Geologic time
Genetic methods
Linguistic methods
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In wide use
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limited use
By specialty
Displays and
Year naming

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