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ISO 8601 is an
international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteris ...
covering the worldwide exchange and communication of date- and
time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the duration of events or the intervals between them, and to of ...

time
-related data. It is maintained by the
Geneva , neighboring_municipalities= Carouge Carouge () is a Municipalities of Switzerland, municipality in the Canton of Geneva, Switzerland. History Carouge is first mentioned in the Early Middle Ages as ''Quadruvium'' and ''Quatruvio''. In 1248 ...

Geneva
-based
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task ...
(ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019. The standard aims to provide a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times. In general, ISO 8601 applies to these representations and formats: ''dates,'' in the Gregorian calendar (including the proleptic Gregorian calendar); ''times,'' based on the 24-hour timekeeping system, with optional
UTC offset The UTC offset is the difference in hours and minutes between Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time pas ...
; ''time intervals''; and combinations thereof.ISO 8601:2004 section 1 Scope The standard does not assign specific meaning to any element of the dates/times represented: the meaning of any element depends on the context of its use. Dates and times represented cannot use words that do not have a specified numerical meaning within the standard (thus excluding names of years in the
Chinese calendar The traditional Chinese calendar (officially known as the Agricultural Calendar [], Former Calendar [], Traditional Calendar [] or Yin Calendar []), is a lunisolar calendar which reckons years, months and days according to astronomical pheno ...
), or that do not use computer characters (excludes images or sounds). In representations that adhere to the ISO 8601 ''interchange standard'', dates and times are arranged such that the greatest temporal term (typically a year) is placed at the left and each successively lesser term is placed to the right of the previous term. Representations must be written in a combination of
Arabic numerals Arabic numerals are the ten numerical digit A numerical digit (often shortened to just digit) is a single symbol used alone (such as "2") or in combinations (such as "25"), to represent numbers in a Positional notation, positional numeral sy ...

Arabic numerals
and the specific computer characters (such as "-", ":", "T", "W", "Z") that are assigned specific meanings within the standard; that is, such commonplace descriptors of dates (or parts of dates) as "January", "Thursday", or "New Year's Day" are not allowed in interchange representations within the standard.


History

The first edition of the ISO 8601 standard was published as ''ISO 8601:1988'' in 1988. It unified and replaced a number of older ISO standards on various aspects of date and time notation:
ISO 2014 ISO 2014 is an international standard that was issued in April 1976, and superseded by ISO 8601 ISO 8601 is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or re ...
,
ISO 2015ISO 2015 is a standard of the International Organization for Standardization, International Organization for Standardization (ISO), superseded by ISO standard ISO 8601. The standard ISO week date, ISO week numbering system was introduced in ISO 2015. ...
,
ISO 2711 ISO 2711 is an standard describing formats for . It was issued in 1973, and was superseded by in June 1988. References #02711 {{measurement-stub ...
,
ISO 3307 ISO 3307 is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, ...
, and
ISO 4031 ISO 4031 is a superseded international standard An international standard is a technical standard developed by one or more international standards organization, standards organizations. International standards are available for consideration and u ...
. It has been superseded by a second edition ''ISO 8601:2000'' in 2000, by a third edition ''ISO 8601:2004'' published on 1 December 2004, and withdrawn and revised by ''ISO 8601-1:2019'' and ''ISO 8601-2:2019'' on 25 February 2019. ISO 8601 was prepared by, and is under the direct responsibility of, ISO Technical Committee TC 154. ISO 2014, though superseded, is the standard that originally introduced the all-numeric date notation in most-to-least-significant order . The ISO week numbering system was introduced in ISO 2015, and the identification of days by ordinal dates was originally defined in ISO 2711. Issued in February 2019, the fourth revision of the standard ISO 8601-1:2019 represents slightly updated contents of the previous ISO 8601:2004 standard, whereas the new ISO 8601-2:2019 defines various extensions such as uncertainties or parts of the #Extended Date/Time Format (EDTF).


General principles

*Date and time values are ordered from the largest to smallest unit of time: year, month (or week), day, hour, minute, second, and fraction of second. The
lexicographical order In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
of the representation thus corresponds to chronological order, except for date representations involving negative years or time offset. This allows dates to be naturally by, for example, file systems. *Each date and time value has a fixed number of digits that must be padded with
leading zero In typography, leading ( ) is the space between adjacent lines of type; the exact definition varies. In hand typesetting, leading is the thin strips of lead (or aluminium) that were inserted between lines of type in the composing stick to incr ...
s. *Representations can be done in one of two formatsa basic format with a minimal number of separators or an extended format with separators added to enhance human readability.ISO, FAQ:
Numeric representation of Dates and Time
'
The standard notes that "The basic format should be avoided in
plain text In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...

plain text
." The separator used between date values (year, month, week, and day) is the
hyphen The hyphen is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
, while the
colon Colon commonly refers to: * Colon (punctuation) (:), a punctuation mark * Major part of large intestine, the final section of the digestive system Colon may also refer to: Places * Colon, Michigan, US * Colon, Nebraska, US * Kowloon, Hong Kong, s ...

colon
is used as the separator between time values (hours, minutes, and seconds). For example, the 6th day of the 1st month of the year 2009 may be written as in the extended format or simply as "20090106" in the basic format without ambiguity. *For reduced precision,Earlier versions of ISO 8601 used the word ''accuracy'', not ''precision'', in the relevant section, e.g: 2.3.7 representation with reduced accuracy. This was corrected in ISO 8601-1:2019. any number of values may be dropped from any of the date and time representations, but in the order from the least to the most significant. For example, "2004-05" is a valid ISO 8601 date, which indicates May (the fifth month) 2004. This format will never represent the 5th day of an unspecified month in 2004, nor will it represent a time-span extending from 2004 into 2005. *If necessary for a particular application, the standard supports the addition of a
decimal fraction The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is t ...
to the smallest time value in the representation.


Dates

The standard uses the
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
, which "serves as an international standard for civil use." ISO 8601:2004 fixes a reference calendar date to the Gregorian calendar of 20 May 1875 as the date the (
Metre Convention The Metre Convention (french: link=no, Convention du Mètre), also known as the Treaty of the Metre, is an international treaty A treaty is a formal, legally binding written agreement between actors in international law International ...

Metre Convention
) was signed in
Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,175,601 residents , in an area of more than . Since the 17th century, Paris ha ...

Paris
(the explicit reference date was removed in ISO 8601-1:2019). However, ISO calendar dates before the convention are still compatible with the Gregorian calendar all the way back to the official introduction of the Gregorian calendar on 15 October 1582. Earlier dates, in the
proleptic Gregorian calendar The proleptic Gregorian calendar is produced by extending the Gregorian calendar The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian ca ...
, may be used by mutual agreement of the partners exchanging information. The standard states that every date must be consecutive, so usage of the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman Roman or Romans usually refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century B ...
would be contrary to the standard (because at the switchover date, the dates would not be consecutive).


Years

ISO 8601 prescribes, as a minimum, a four-digit year YYYto avoid the
year 2000 problem The Year 2000 problem, also known as the Y2K problem, Millennium bug, Y2K bug, Y2K glitch or Y2K error, refers to potential computer errors related to the formatting and storage of calendar data for dates in and after the year 2000 20 ...
. It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others . However, years before
1583 __NOTOC__ 1583 (Roman numerals, MDLXXXIII) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Tuesday of the Julian calendar. As of the start of 1583, the Gregorian calendar was 10 days ahead of the J ...
are not automatically allowed by the standard. Instead "values in the range through
582 __NOTOC__ Year 582 ( DLXXXII) was a common year starting on Thursday A common year starting on Thursday is any non-leap year A leap year (also known as an intercalary year or wikt:bissextile, bissextile year) is a calendar year that contains ...
shall only be used by mutual agreement of the partners in information interchange." To represent years before 0000 or after 9999, the standard also permits the expansion of the year representation but only by prior agreement between the sender and the receiver. An expanded year representation YYYYYmust have an agreed-upon number of extra year digits beyond the four-digit minimum, and it must be prefixed with a + or − sign instead of the more common AD/BC (or CE/BCE) notation; by convention 1 BC is labelled +0000, 2 BC is labeled −0001, and so on.


Calendar dates

Calendar date representations are in the form shown in the adjacent box. YYYindicates a four-digit year, 0000 through 9999. indicates a two-digit month of the year, 01 through 12. indicates a two-digit day of that month, 01 through 31. For example, "5 April 1981" may be represented as either 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 to mean "1981 April". The 2000 version allowed writing to mean "April 5" but the 2004 version does not allow omitting the year when a month is present. One may simply write "1981" to refer to that year, "198" to refer to the
decade A decade is a period of 10 years. The word is derived (via French and Latin) from the grc, δεκάς, dekas, which means ''a group of ten''. Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupin ...
from 1980 to 1989 inclusive, or "19" to refer to the
century A century is a period of 100 years. Centuries are numbered names of numbers in English#Ordinal numbers, ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is s ...

century
from 1900 to 1999 inclusive. Although the standard allows both the and YYYYMMDD formats for complete calendar date representations, if the day is omitted then only the format is allowed. By disallowing dates of the form YYYYMM, the standard avoids confusion with the truncated representation YYMMDD (still often used).


Week dates

Week date representations are in the formats as shown in the adjacent box. YYYindicates the ''ISO week-numbering year'' which is slightly different from the traditional
Gregorian calendar The Gregorian calendar is the calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, speci ...
year (see below). wwis the ''
week number 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 wee ...
'' prefixed by the letter ''W'', from W01 through W53. 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 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 - 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: * is written "" * is written ""


Ordinal dates

An
ordinal date An ordinal date is a calendar date typically consisting of a ''year'' and a ''day of the year'' ranging between 1 and 366 (starting on January 1), though year may sometimes be omitted. The two numbers can be formatted as YYYY-DDD to comply with t ...
is a simple form 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. As represented above, YYYindicates a year. is the day of that year, from 001 through 365 (366 in
leap year A leap year (also known as an intercalary year or wikt:bissextile, bissextile year) is a calendar year that contains an additional day (or, in the case of a lunisolar calendar, a month) added to keep the calendar year synchronized with the astron ...

leap year
s). For example, is also . 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 The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field ou ...
, a sequential count of the number of days since day 0 beginning Greenwich noon, Julian proleptic calendar (or noon on ISO date which uses the Gregorian proleptic calendar with a year 0000).


Times

ISO 8601 uses the
24-hour clock The 24-hour clock, popularly referred to in the United States and some other countries as military time, is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hour ...
system. As of ISO 8601-1:2019, the ''basic format'' is Tmm]and the ''extended format'' is T Earlier versions omitted the T (representing time) in both formats. *refers to a zero-padded
hour An hour (symbol A symbol is a mark, sign, or that indicates, signifies, or is understood as representing an , , or . Symbols allow people to go beyond what is n or seen by creating linkages between otherwise very different s and s. All ...
between 00 and 23. *refers to a zero-padded
minute The minute is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piec ...
between 00 and 59. *refers to a zero-padded
second The second (symbol: s, also abbreviated: sec) is the base unit of time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, th ...
between 00 and 60 (where 60 is only used to denote an added
leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

leap second
). So a time might appear as either "T134730" in the ''basic format'' or "T13:47:30" in the ''extended format''. ISO 8601-1:2019 allows the T to be omitted in the extended format, as in "13:47:30", but only allows the T to be omitted in the basic format when there is no risk of ambiguity with date expressions. Either the seconds, or the minutes and seconds, may be omitted from the basic or extended time formats for greater brevity but decreased precision; the resulting reduced precision time formats are: *Tmm] in ''basic format'' or in ''extended format'', when seconds are omitted. *T when both seconds and minutes are omitted. As of ISO 8601-1:2019 ''
midnight Midnight is the transition time from one day A day is approximately the period during which the Earth completes one rotation around its axis, which takes around 24 hours. A solar day is the length of time which elapses between the Sun reachin ...

midnight
'' may only be referred to as "00:00", corresponding to the beginning of a calendar day. Earlier versions of the standard allowed "24:00" corresponding to the end of a day, but this is explicitly disallowed by the 2019 revision. A decimal fraction may be added to the lowest order time element present, in any of these representations. A
decimal mark A decimal separator is a symbol used to separate the integer An integer (from the Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the ar ...
, either a
comma The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
or a (following
ISO 80000-1ISO 80000-1:2009 is a standard describing scientific and mathematical quantities and their units. The standard, whose full name is ''Quantities and units Part 1: General'' was developed by the International Organization for Standardization The ...
according to ISO 8601:1-2019, which does not stipulate a preference except within International Standards, but with a preference for a
comma The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...
according to ISO 8601:2004) is used as a separator between the time element and its fraction. To denote "14 hours, 30 and one half minutes", do not include a seconds figure. Represent it as "14:30,5", "T1430,5", "14:30.5", or "T1430.5". There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the
precision Precision, precise or precisely may refer to: Science, and technology, and mathematics Mathematics and computing (general) * Accuracy and precision, measurement deviation from true value and its scatter * Significant figures, the number of digits ...
of a decimal fraction is 3 for a DATETIME, i.e., "yyyy-mm-ddThh:mm:ss mmm.


Time zone designators

Time zone A time zone is an area that observes a uniform standard time Standard time is the synchronization of clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invent ...
s in ISO 8601 are represented as local time (with the location unspecified), as UTC, or as an offset from UTC.


Local time (unqualified)

If no UTC relation information is given with a time representation, the time is assumed to be in local time. While it ''may'' be safe to assume local time when communicating in the same time zone, it is ambiguous when used in communicating across different time zones. Even within a single geographic time zone, some local times will be ambiguous if the region observes
daylight saving time#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...

daylight saving time
. It is usually preferable to indicate a time zone (zone designator) using the standard's notation.


Coordinated Universal Time (UTC)

If the time is in UTC, add a ''Z'' directly after the time without a space. ''Z'' is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "T0930Z". "14:45:15 UTC" would be "14:45:15Z" or "T144515Z". The ''Z'' suffix in the ISO 8601 time representation is sometimes referred to as "Zulu time" because the same letter is used to designate the Zulu time zone. However the ACP 121 standard that defines the list of military time zones makes no mention of UTC and derives the "Zulu time" from the
Greenwich Mean Time Greenwich Mean Time (GMT) is the Local mean time, mean solar time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, i ...
which was formerly used as the international civil time standard. GMT is no longer precisely defined by the scientific community and can refer to either UTC or
UT1 Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time Coordinated Universal Time or UTC is t ...
depending on context.


Time offsets from UTC

The
UTC offset The UTC offset is the difference in hours and minutes between Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time pas ...
is appended to the time in the same way that 'Z' was above, in the form ± ±mm], or ± Negative
UTC offset The UTC offset is the difference in hours and minutes between Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time pas ...
s describe a
time zone A time zone is an area that observes a uniform standard time Standard time is the synchronization of clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invent ...
west of UTC±00:00, where the
civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time Standard time is the synchronization of clock A clock ...
is behind (or earlier) than UTC so the zone designator will look like "−03:00","−0300", or "−03". Positive
UTC offset The UTC offset is the difference in hours and minutes between Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time pas ...
s describe a
time zone A time zone is an area that observes a uniform standard time Standard time is the synchronization of clock A clock or a timepiece is a device used to Measurement, measure and indicate time. The clock is one of the oldest Invent ...
at or east of UTC±00:00, where the
civil time In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time Standard time is the synchronization of clock A clock ...
is the same as or ahead (or later) than UTC so the zone designator will look like "+02:00","+0200", or "+02". Examples * "−05:00" for New York on standard time ( UTC-05:00) * "−04:00" for New York on daylight saving time ( UTC-04:00) * "+00:00" (but not "−00:00") for London on standard time ( UTC±00:00) * "+02:00" for Cairo ( UTC+02:00) * "+05:30" for Mumbai ( UTC+05:30) * "+14:00" for Kiribati ( UTC+14:00) See
List of UTC time offsets This is a list of the UTC time offsets, showing the difference in hours and minutes from Coordinated Universal Time (UTC), from the westernmost (−12:00) to the easternmost (+14:00). It includes countries and regions that observe them during stand ...
for other
UTC offset The UTC offset is the difference in hours and minutes between Coordinated Universal Time Coordinated Universal Time or UTC is the primary time standard A time standard is a specification for measuring time: either the rate at which time pas ...
s. To represent a negative offset, ISO 8601 specifies using a
minus sign The plus and minus signs, and , are mathematical symbol A mathematical symbol is a figure or a combination of figures that is used to represent a mathematical object A mathematical object is an abstract concept arising in mathematics. In the us ...
. If the interchange character set is limited and does not have a minus sign character, then the
hyphen-minus The hyphen-minus is the most commonly used type of hyphen The hyphen is a punctuation mark used to join word In linguistics, a word of a spoken language can be defined as the smallest sequence of phonemes that can be uttered in isolatio ...
should be used.
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding Character encoding is the process of assigning numbers to graphical Graphics (from Greek Greek may refer to: Greece Anything of, ...
does not have a minus sign, so its hyphen-minus character (code is 45 decimal or 2D hexadecimal) would be used. If the character set has a minus sign, then that character should be used.
Unicode Unicode, formally the Unicode Standard, is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expressed in most of the world's wri ...

Unicode
has a minus sign, and its character code is U+2212 (2212 hexadecimal); the HTML character entity invocation is −. The following times all refer to the same moment: "18:30Z", "22:30+04", "1130−0700", and "15:00−03:30".
Nautical time Nautical time is a system devised to allow ships on high seas to express their local time. Nautical time zones are split into one hour intervals for every 15 degree change in a ship's longitudinal coordinate. Nautical time keeping dates back to t ...
zone letters are not used with the exception of Z. To calculate UTC time one has to subtract the offset from the local time, e.g. for "15:00−03:30" do 15:00 − (−03:30) to get 18:30 UTC. An offset of zero, in addition to having the special representation "Z", can also be stated numerically as "+00:00", "+0000", or "+00". However, it is not permitted to state it numerically with a negative sign, as "−00:00", "−0000", or "−00". The section dictating sign usage states that a plus sign must be used for a positive or zero value, and a minus sign for a negative value. Contrary to this rule, RFC 3339, which is otherwise a profile of ISO 8601, permits the use of "-00", with the same denotation as "+00" but a differing connotation.


Combined date and time representations

A single point in time can be represented by concatenating a complete date expression, the letter ''"T"'' as a delimiter, and a valid time expression. For example, . In ISO 8601:2004 it was permitted to omit the ''"T"'' character by mutual agreement as in , but this provision was removed in ISO 8601-1:2019. Separating date and time parts with other characters such as space is not allowed in ISO 8601, but allowed in its profile RFC 3339. If a time zone designator is required, it follows the combined date and time. For example, or . Either basic or extended formats may be used, but both date and time must use the same format. The date expression may be calendar, week, or ordinal, and must use a complete representation. The time may be represented using a specified reduced precision format.


Durations

Durations define the amount of intervening time in a time interval and are represented by the format P T or P as shown on the aside. In these representations, the is replaced by the value for each of the date and time elements that follow the Leading zeros are not required, but the maximum number of digits for each element should be agreed to by the communicating parties. The capital letters ''P'', ''Y'', ''M'', ''W'', ''D'', ''T'', ''H'', ''M'', and ''S'' are designators for each of the date and time elements and are not replaced. *''P'' is the duration designator (for ''period'') placed at the start of the duration representation. **''Y'' is the year designator that follows the value for the number of years. **''M'' is the month designator that follows the value for the number of months. **''W'' is the week designator that follows the value for the number of weeks. **''D'' is the day designator that follows the value for the number of days. *''T'' is the time designator that precedes the time components of the representation. **''H'' is the hour designator that follows the value for the number of hours. **''M'' is the minute designator that follows the value for the number of minutes. **''S'' is the second designator that follows the value for the number of seconds. For example, "P3Y6M4DT12H30M5S" represents a duration of "three years, six months, four days, twelve hours, thirty minutes, and five seconds". Date and time elements including their designator may be omitted if their value is zero, and lower-order elements may also be omitted for reduced precision. For example, "P23DT23H" and "P4Y" are both acceptable duration representations. However, at least one element must be present, thus "P" is not a valid representation for a duration of 0 seconds. "PT0S" or "P0D", however, are both valid and represent the same duration. To resolve ambiguity, "P1M" is a one-month duration and "PT1M" is a one-minute duration (note the time designator, T, that precedes the time value). The smallest value used may also have a decimal fraction, as in "P0.5Y" to indicate half a year. This decimal fraction may be specified with either a
comma The comma is a punctuation Punctuation (or sometimes interpunction) is the use of spacing, conventional signs (called punctuation marks), and certain typographical devices as aids to the understanding and correct reading of written text, ...

comma
or a
full stop The full stop (Commonwealth English The use of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the ...

full stop
, as in "P0,5Y" or "P0.5Y". The standard does not prohibit date and time values in a duration representation from exceeding their "carry over points" except as noted below. Thus, "PT36H" could be used as well as "P1DT12H" for representing the same duration. But keep in mind that "PT36H" is not the same as "P1DT12H" when switching from or to
Daylight saving time#REDIRECT Daylight saving time Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by ...

Daylight saving time
. Alternatively, a format for duration based on combined date and time representations may be used by agreement between the communicating parties either in the basic format PYYYYMMDDThhmmss or in the extended format . For example, the first duration shown above would be . However, individual date and time values cannot exceed their moduli (e.g. a value of 13 for the month or 25 for the hour would not be permissible). Although the standard describes a duration as part of time intervals, which are discussed in the next section, the duration format (or a subset thereof) is widely used independent of time intervals, as with the Java 8 Duration class.


Time intervals

A time interval is the intervening time between two time points. The amount of intervening time is expressed by a duration (as described in the previous section). The two time points (start and end) are expressed by either a combined date and time representation or just a date representation. There are four ways to express a time interval: #Start and end, such as "2007-03-01T13:00:00Z/2008-05-11T15:30:00Z" #Start and duration, such as "2007-03-01T13:00:00Z/P1Y2M10DT2H30M" #Duration and end, such as "P1Y2M10DT2H30M/2008-05-11T15:30:00Z" #Duration only, such as "P1Y2M10DT2H30M", with additional context information Of these, the first three require two values separated by an ''interval designator'' which is usually a solidus (more commonly referred to as a forward slash "/"). Section 3.2.6 of ISO 8601-1:2019 notes that "A solidus may be replaced by a double hyphen --"by mutual agreement of the communicating partners", and previous versions used notations like "2000--2002". Use of a double hyphen instead of a solidus allows inclusion in computer
filename A filename or file name is a name used to uniquely identify a computer file A computer file is a computer resource for recording data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more techni ...

filename
s; in common operating systems, a solidus is a
reserved character A filename or file name is a name used to uniquely identify a computer file A computer file is a computer resource for recording data Data are units of information Information can be thought of as the resolution of uncertainty; i ...
and is not allowed in a filename. For / expressions, if any elements are missing from the end value, they are assumed to be the same as for the start value including the time zone. This feature of the standard allows for concise representations of time intervals. For example, the date of a two-hour meeting including the start and finish times could be simply shown as "2007-12-14T13:30/15:30", where "/15:30" implies "/2007-12-14T15:30" (the same date as the start), or the beginning and end dates of a monthly billing period as "2008-02-15/03-14", where "/03-14" implies "/2008-03-14" (the same year as the start). If greater precision is desirable to represent the time interval, then more time elements can be added to the representation. An interval denoted can start at any time on and end at any time on , whereas includes the start and end times. To explicitly include all of the start and end dates, the interval would be represented as .


Repeating intervals

Repeating intervals are specified in clause "4.5 Recurring time interval". They are formed by adding "R " to the beginning of an interval expression, where ''R'' is used as the letter itself and is replaced by the number of repetitions. Leaving out the value for or specifying a value of -1, means an unbounded number of repetitions. A value of 0 for means the interval is not repeated. If the interval specifies the start (forms 1 and 2 above), then this is the start of the repeating interval. If the interval specifies the end but not the start (form 3 above), then this is the end of the repeating interval. For example, to repeat the interval of "P1Y2M10DT2H30M" five times starting at , use .


Truncated representations

ISO 8601:2000 allowed truncation (by agreement), where leading components of a date or time are omitted. Notably, this allowed two-digit years to be used and the ambiguous formats YY-MM-DD and YYMMDD. This provision was removed in ISO 8601:2004. Only the first type (specific date in the implied century) omits the leading - for century. All other formats have one leading - per omitted century, year, and month.


Standardised extensions

ISO 8601-2:2019 defines a set of standardised extensions to the ISO 8601 date and time formats. ; Extended Date/Time Format (EDTF) : The EDTF is given as an example of a profile of ISO 8601. Some of its features are: :* and approximate qualifiers, '?' and '~', as well as their combined used, '%'; they can be applied to the whole date or to individual components. :* Time intervals with an open (unbounded) end or an unknown end. :* Exponential and significant figure notation in years. :* Special "month" values indicating sub-year groupings such as seasons and quarters. :* Syntax for serializing a list of dates. : The EDTF features are described in the "Date and Time Extensions" section of ISO 8601-2:2019. ; Repeat rules for recurring time intervals : ISO 8601-2:2019 also defines a format to constrain repeating intervals based on syntax from
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.


Usage

On the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
, the
World Wide Web Consortium The World Wide Web Consortium (W3C) is the main international standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary ...
(W3C) uses the
IETF The Internet Engineering Task Force (IETF) is an open standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary functi ...
standard based on ISO 8601 in defining a profile of the standard that restricts the supported date and time formats to reduce the chance of error and the complexity of software. The very simple specification is based on a draft of the RFC 3339 mentioned below. ISO 8601 is referenced by several specifications, but the full range of options of ISO 8601 is not always used. For example, the various
electronic program guide Electronic programming guides (EPGs) and interactive programming guides (IPGs) are menu-based systems that provide users of television, radio Radio is the technology of signaling and telecommunication, communicating using radio waves. R ...

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standards for TV, digital radio, etc. use several forms to describe points in time and durations. The
ID3 ID3 is a metadata Metadata is "data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sense, data are a set of values of qualitative property, qualitative or quantity, quantitative ...
audio meta-data specification also makes use of a subset of ISO 8601. The X.690 encoding standard's GeneralizedTime makes use of another subset of ISO 8601.


Commerce

The ISO 8601 week date, as of 2006, appeared in its basic form on major brand commercial packaging in the United States. Its appearance depended on the particular packaging, canning, or bottling plant more than any particular brand. The format is particularly useful for quality assurance, so that production errors can be readily traced to work weeks, and products can be correctly targeted for recall.


RFCs

IETF RFC 3339 defines a profile of ISO 8601 for use in Internet protocol suite, Internet protocols and Internet standard, standards. It explicitly excludes durations and dates before the common era. The more complex formats such as week numbers and ordinal days are not permitted. RFC 3339 deviates from ISO 8601 in allowing a zero time zone offset to be specified as "-00:00", which ISO 8601 forbids. RFC 3339 intends "-00:00" to carry the connotation that it is not stating a preferred time zone, whereas the conforming "+00:00" or any non-zero offset connotes that the offset being used is preferred. This convention regarding "-00:00" is derived from earlier RFCs, such as RFC 2822 which uses it for timestamps in email headers. RFC 2822 made no claim that any part of its timestamp format conforms to ISO 8601, and so was free to use this convention without conflict.


Adoption as national standards


See also

*Astronomical year numbering *Date and time representation by country * Date format by country * Horology


Notes and references


External links


ISO's catalog entry for ISO 8601:2004The latest prototype of ISO 8601-1 (ISO/TC 154 N)The latest prototype of ISO 8601-2 (ISO/TC 154 N)Use international date format (ISO) – Quality Web Tips
W3C, The World Wide Web Consortium (W3C) *
ISO 8601 summary by Markus Kuhn
*
W3C Specification about UTC Date and Time
based on ISO 8601:1988 *
IETF The Internet Engineering Task Force (IETF) is an open standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary functi ...
based on ISO 8601:2000
RFC 3339 vs ISO 8601
— Venn diagram illustrating the difference between the two standards. * * Implementation overview
ISO 8601 Implementation Around The World
{{DEFAULTSORT:Iso 8601 Calendaring standards Date and time representation ISO standards, #08601 Specific calendars Time measurement systems