ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019, and an amendment in 2022. The standard provides 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. 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, time intervals, and combinations thereof. 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), 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 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 June 1988. It unified and replaced a number of older ISO standards on various aspects of date and time notation :
ISO 2014: Writing of calendar dates in all-numeric form, published April 1976 following ISO/R 2014 from 1971, originally introduced the hyphenated, most-to-least-significant order notation for Gregorian calendar dates with day of the month as an international standard. ISO 2015: Numbering of weeks, also from April 1976 with ISO R 2015 from 1971, first specified the rules for determining the ordinal number of a calendar week in a year and a day within a week. ISO 2711: Representation of ordinal dates, issued in January 1973, introduced the notation for the day of the year, which effectively treats only the final, 366th day of leap years differently. ISO 3307: Representation of time of the day, dated March 1975, standardized 24-hour wall clock times and their notation, but without handling time zones. ISO 4031: Representation of time of the day, published December 1978, added numerical offsets from UTC (or GMT back then). This first, unified edition was superseded by a second edition, ISO 8601:2000, in 2000, that by a revised, third edition, ISO 8601:2004, published on 1 December 2004, and that by ISO 8601-1:2019 and ISO 8601-2:2019 on 25 February 2019. All editions and parts of ISO 8601 and its predecessors were prepared by, and are under the direct responsibility of, ISO Technical Committee TC 154. 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, but is now the first of two parts. The other part, ISO 8601-2:2019, is entirely new. It defines various extensions such as uncertainties or parts of the Extended Date/Time Format (EDTF). An amendment to ISO 8601-1 was published in October 2022 featuring minor technical clarifications and attempts to remove ambiguities in definitions. The most significant change, however, was the reintroduction of the "24:00:00" format to refer to the instant at the end of a calendar day. An amendment to ISO 8601-2 was published in January 2025.
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 of the representation thus corresponds to chronological order, except for date representations involving negative years or time offset. This allows dates to be naturally sorted by, for example, file systems. Each date and time value has a fixed number of digits that must be padded with leading zeros. 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, while the 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 "2009-01-06" in the extended format or as "20090106" in the basic format without ambiguity. For reduced precision, 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.
Dates
The standard uses the Gregorian calendar, which "serves as an international standard for civil use". ISO 8601 allows Gregorian dates from the introduction of the calendar on 15 October 1582. For earlier (pre-Gregorian) dates, the calendar may be extended before its introduction (the proleptic Gregorian calendar) by explicit agreement of the parties involved. Such proleptic dates may not be adjusted to reconcile them with Julian dates.
Years
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