ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard
International standard
International standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...

 covering the exchange of date
Calendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...

 and time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

-related data. It was issued by the International Organization for Standardization
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

 (ISO) and was first published in 1988. The purpose of this standard is to provide an unambiguous and well-defined method of representing dates and times, so as to avoid misinterpretation of numeric representations of dates and times, particularly when data is transferred between countries with different conventions
Date and time notation by country
Different conventions exist around the world for date and time representation, both written and spoken.- Differences :Differences can exist in:*The calendar that is used.*The order in which the year, month and day are represented....

 for writing numeric dates and times.

The standard organizes the data so the largest temporal term (the year) appears first in the data string and progresses to the smallest term (the second). It also provides for a standardized method of communicating time-based information across time zones by attaching an offset to Coordinated Universal Time (UTC).
Date and time (current at page generation)
expressed according to ISO 8601:
Separate date and time in UTC:
Combined date and time in UTC:
Date with week number:
Ordinal date: -


The first edition of the ISO 8601 standard was published in 1988. It unified and replaced a number of older ISO standards on various aspects of date and time notation: ISO 2014
ISO 2014
ISO 2014 is a standard superseded by ISO 8601. ISO 2014 was the standard that originally introduced the big-endian all-numeric date notation [YYYY]-[MM]-[DD]. It was issued as an international standard in 1976, technically identical to ISO Recommendation R 2014 from 1971....

, ISO 2015
ISO 2015
ISO 2015 is a standard superseded by ISO 8601. The ISO week numbering system was introduced in ISO 2015. It was issued as an international standard in 1976, technically identical to ISO Recommendation R 2015 from 1971....

, ISO 2711
ISO 2711
ISO 2711 is a standard superseded by ISO 8601. Identification of days by ordinal dates was originally defined in ISO 2711. It was issued in 1973....

, ISO 3307
ISO 3307
ISO 3307 is an international standard for date and time representations issued by the International Organization for Standardization . The standard was issued in 1975, then was superseded by ISO 8601 in 1988....

, and ISO 4031
ISO 4031
ISO 4031 is a standard superseded by ISO 8601. Representations of local time differentials were originally specified in ISO 4031. The standard was issued in 1978....

. It has been superseded by a second edition in 2000 and by the current third edition published on 3 December 2004.

ISO 2014 was the standard that originally introduced the big-endian
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...

 all-numeric date notation . The ISO week numbering system was introduced in ISO 2015, and the identification of days by ordinal dates was originally defined in ISO 2711.

It is maintained by ISO Technical Committee TC 154.

General principles

  • Date and time values are organized from the most to the least significant: year, month (or week), day, hour, minute, second, and fraction of second. The lexicographical order
    Lexicographical order
    In mathematics, the lexicographic or lexicographical order, , is a generalization of the way the alphabetical order of words is based on the alphabetical order of letters.-Definition:Given two partially ordered sets A and B, the lexicographical order on...

     of the representation thus corresponds to chronological order, except for date representations involving negative years. This allows dates to be naturally sorted
    Sorting is any process of arranging items in some sequence and/or in different sets, and accordingly, it has two common, yet distinct meanings:# ordering: arranging items of the same kind, class, nature, etc...

     by, for example, file systems.
  • Each date and time value has a fixed number of digits that must be padded with leading zero
    Leading zero
    A leading zero is any 0 digits, that lead a number string in a positional notation. For example, James Bond's famous identifier, 007, has two leading zeros. Leading zeros occupy most significant digits, which could be left blank or omitted for the same numeric value...

  • Representations can be done in one of two formats—a basic format with a minimal number of separators or an extended format with separators added to enhance human readability. The standard notes that "The basic format should be avoided in plain text." The separator used between date values (year, month, week, and day) is the hyphen
    The hyphen is a punctuation mark used to join words and to separate syllables of a single word. The use of hyphens is called hyphenation. The hyphen should not be confused with dashes , which are longer and have different uses, or with the minus sign which is also longer...

    , while the colon
    Colon (punctuation)
    The colon is a punctuation mark consisting of two equally sized dots centered on the same vertical line.-Usage:A colon informs the reader that what follows the mark proves, explains, or lists elements of what preceded the mark....

    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. The extended formats are preferred over the basic formats not only for human readability, but because some basic formats can appear to be ambiguous to those unfamiliar with the standard.
  • For reduced accuracy, 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 to the smallest time value in the representation.


The standard uses the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

, and serves as an international standard for civil use.

ISO 8601 fixes a reference calendar date to the Gregorian calendar of as the date the Convention du Mètre (Metre Convention) was signed in Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

. 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 . Earlier dates, in the proleptic Gregorian calendar
Proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582.-Usage:...

, 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
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 would be contrary to the standard (because at the switchover date, the dates would not be consecutive).



ISO 8601 prescribes, as a minimum, a four-digit year [YYYY] to avoid the year 2000 problem
Year 2000 problem
The Year 2000 problem was a problem for both digital and non-digital documentation and data storage situations which resulted from the practice of abbreviating a four-digit year to two digits.In computer programs, the practice of representing the year with two...

. It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD.

To represent years before 0000
Year zero
"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...

 or after 9999
Year 10,000 problem
The Year 10,000 problem is the class of all potential software bugs that would emerge when the need to express years with five digits arises...

, 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 [±YYYYY] must have an agreed-upon number of extra year digits beyond the four-digit minimum, and it should always be prefixed with a + or − sign instead of the common AD
Anno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....

Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...

 or BC/BCE notation; by convention year zero
Year zero
"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...

 is labelled positive: +0000. Note the addition of a year zero causes earlier years to be different by one when converted; for example, the year 3 BC would be denoted by −0002.

Calendar dates

YYYY-MM (but not YYYYMM)

Calendar date representations are in the form as shown in the box to the right. [YYYY] indicates a four-digit year, 0000 through 9999. [MM] indicates a two-digit month of the year, 01 through 12. [DD] indicates a two-digit day of that month, 01 through 31. For example, "the 5th of 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", and one may simply write "1981" to refer to that year or "19" to refer to the century
A century is one hundred consecutive years. Centuries are numbered ordinally in English and many other languages .-Start and end in the Gregorian Calendar:...

 from 1900 to 1999 inclusive.

Although the standard allows both the YYYY-MM-DD and YYYYMMDD formats for complete calendar date representations, if the day [DD] 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 format as shown in the box to the right. [YYYY] indicates the ISO week-numbering year which is slightly different to the calendar year (see below). [Www] is the week number prefixed by the letter 'W', from W01 through W53. [D] is the weekday number, from 1 through 7, beginning with Monday and ending with Sunday. This form is popular in the manufacturing industries.

There are mutually equivalent descriptions of week 01:
  • the week with the 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.

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 ISO week-numbering year number deviates from the number of the calendar year (Gregorian year) on a Friday, Saturday, and Sunday, or a Saturday and Sunday, or just a Sunday, at the start of the calendar year (which are at the end of the previous ISO week-numbering year) and a Monday, Tuesday and Wednesday, or a Monday and Tuesday, or just a Monday, at the end of the calendar year (which are in week 01 of the next ISO week-numbering year). For Thursdays, the ISO week-numbering year number is always equal to the calendar year number.

  • 2008-12-29 is written ""
  • 2010-01-03 is written ""

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. An advantage is that no separate year numbering like the ISO week-numbering year is needed, while correspondence of lexicographical order and chronological order is preserved.

Ordinal dates


An ordinal date
Ordinal date
An ordinal date is a calendar date typically consisting of a year and a day of year ranging between 1 and 366 , though year may sometimes be omitted...

 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, [YYYY] indicates a year. [DDD] is the day of that year, from 001 through 365 (366 in leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal 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 incorrectly referred to as "Julian Date", whereas the astronomical Julian Date
Julian day
Julian day is used in the Julian date system of time measurement for scientific use by the astronomy community, presenting the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon...

 is 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]).


hh:mm:ss or hhmmss
hh:mm or hhmm

ISO 8601 uses the 24-hour clock
24-hour clock
The 24-hour clock is a convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today...

 system. The basic format is [hh][mm][ss] and the extended format is [hh]:[mm]:[ss].
  • [hh] refers to a zero-padded hour
    The hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...

     between 00 and 24 (where 24 is only used to notate midnight at the end of a calendar day).
  • [mm] refers to a minute
    A minute is a unit of measurement of time or of angle. The minute is a unit of time equal to 1/60th of an hour or 60 seconds. In the UTC time scale, a minute on rare occasions has 59 or 61 seconds; see leap second. The minute is not an SI unit; however, it is accepted for use with SI units...

     between 00 and 59.
  • [ss] refers to a second
    The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

     between 00 and 60 (where 60 is only used to notate an added leap second
    Leap second
    A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...


So a time might appear as either "134730" in the basic format or "13:47:30" in the extended format.

It is also acceptable to omit lower order time elements for reduced accuracy: [hh]:[mm], [hh][mm] and [hh] are all used.

Midnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....

is a special case and can be referred to as both "00:00" and "24:00". The notation "00:00" is used at the beginning of a calendar day and is the more frequently used. At the end of a day use "24:00". Note that "2007-04-05T24:00" is the same instant as "2007-04-06T00:00" (see Combined date and time representations below).

Decimal fractions may also be added to any of the three time elements. A decimal point, either a comma
Comma (punctuation)
The comma is a punctuation mark. It has the same shape as an apostrophe or single closing quotation mark in many typefaces, but it differs from them in being placed on the baseline of the text. Some typefaces render it as a small line, slightly curved or straight but inclined from the vertical, or...

 or a dot
Full stop
A full stop is the punctuation mark commonly placed at the end of sentences. In American English, the term used for this punctuation is period. In the 21st century, it is often also called a dot by young people...

 (without any preference as stated most recently in resolution 10 of the 22nd General Conference CGPM in 2003), is used as a separator between the time element and its fraction. A fraction may only be added to the lowest order time element in the representation. To denote "14 hours, 30 and one half minutes", do not include a seconds figure. Represent it as "14:30,5", "1430,5", "14:30.5", or "1430.5". 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.

Time zone designators

There are no time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

 designators in ISO 8601. Time is only represented as local time or in relation to UTC.

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


If the time is in UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

, 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 "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".

UTC time is also known as 'Zulu' time, since 'Zulu' is the NATO phonetic alphabet word for 'Z'.

Time offsets from UTC

The offset from UTC is given in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...

 during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

, e.g. a time offset in Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...

, would be "-06:00" for the winter (Central Standard Time) and "-05:00" for the summer (Central Daylight Time).

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
The establishment of nautical standard times, nautical standard time zones and the nautical date line were recommended by the Anglo-French Conference on Time-keeping at Sea in 1917. The Conference recommended that the standard apply to all ships, both military and civilian...

 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.

The offset can also be used in the case where the UTC time is known, but the local offset is not. In this case the offset is "-00:00", which is semantically different from "Z" or "+00:00", as these imply that UTC is the preferred reference point for those times.

Combined date and time representations


Combining date and time representations to represent a single point in time (time point) is quite simple. It is in the format of T