ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date- and time-related data. It was issued by the International Organization for Standardization 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 for writing numeric dates and times.
In general, ISO 8601 applies to representations and formats of dates in the Gregorian calendar, of times based on the 24-hour timekeeping system, of, and combinations thereof. The standard does not assign any specific meaning to elements of the date/time to be represented; the meaning will depend on the context of its use. In addition, dates and times to be represented cannot include words with no specified numerical meaning in the standard or that do not use characters.
In representations for interchange, dates and times are arranged so the largest temporal term is placed to the left and each successively smaller term is placed to the right of the previous term. Representations must be written in a combination of Arabic numerals and certain characters that are given specific meanings within the standard; the implication is that some commonplace ways of writing parts of dates, such as "January" or "Thursday", are not allowed in interchange representations.
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 2015, ISO 2711, ISO 3307, and ISO 4031. It has been superseded by a second edition ISO 8601:2000 in 2000, by a third edition ISO 8601:2004 published on 1 December 2004, and withdrawn and revised by ISO 8601-1:2019 and ISO 8601-2:2019 on 25 February 2019. ISO 8601 was prepared by, and is under the direct responsibility of, ISO Technical Committee TC 154.ISO 2014, though superseded, is the standard that originally introduced the all-numeric date notation in most-to-least-significant order.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.
| Name | Description |
| ISO 8601:1988 | Data elements and interchange formats — Information interchange — Representation of dates and times |
| ISO 8601:1988/COR 1:1991 | Data elements and interchange formats — Information interchange — Representation of dates and times — Technical Corrigendum 1 |
| ISO 8601:2000 | Data elements and interchange formats — Information interchange — Representation of dates and times |
| ISO 8601:2004 | Data elements and interchange formats — Information interchange — Representation of dates and times |
| ISO 8601-1:2019 | Date and time — Representations for information interchange — Part 1: Basic rules |
| ISO 8601-2:2019 | Date and time — Representations for information interchange — Part 2: Extensions |
General principles
- Date and time values are ordered from the largest to smallest unit of time: year, month, 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 formatsa 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 is the hyphen, while the colon is used as the separator between time values. 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 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 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
ISO 8601 fixes a reference calendar date to the Gregorian calendar of 20 May 1875 as the date the Convention du Mètre was signed in Paris. 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, 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 would be contrary to the standard.
Years
ISO 8601 prescribes, as a minimum, a four-digit year to avoid the year 2000 problem. It therefore represents years from 0000 to 9999, year 0000 being equal to 1 BC and all others AD. However, years prior to 1583 are not automatically allowed by the standard. Instead "values in the range through 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 must 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 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. indicates 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 accuracy. 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 or "19" to refer to the 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.
Week dates
| YYYY-Www | or | YYYYWww |
| YYYY-Www-D | or | YYYYWwwD |
Week date representations are in the formats as shown in the adjacent box. indicates the ISO week-numbering year which is slightly different from the traditional Gregorian calendar year. is the week number 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 year's first Thursday in it,
- the week with 4 January in it,
- the first week with the majority of its days in the starting year, and
- the week starting with the Monday in the period 29 December – 4 January.
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 of week 01 and ends at the Sunday before the new ISO year. 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
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, a sequential count of the number of days since day 0 beginning Greenwich noon, Julian proleptic calendar.
Times
ISO 8601 uses the 24-hour clock system. The basic format is and the extended format is ::.- refers to a zero-padded hour between 00 and 24.
- refers to a zero-padded minute between 00 and 59.
- refers to a zero-padded second between 00 and 60.
Either the seconds, or the minutes and seconds, may be omitted from the basic or extended time formats for greater brevity but decreased accuracy; the resulting reduced accuracy time formats are:
- in basic format or : in extended format, when seconds are omitted.
- , when both seconds and minutes are omitted.
Decimal fractions may be added to any of the three time elements. However, a fraction may only be added to the lowest order time element in the representation. A decimal mark, either a comma or a dot 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", "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. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3, i.e., "yyyy-mm-ddThh:mm:ss".
Time zone designators
| <time>Z |
| <time>±hh:mm |
| <time>±hhmm |
| <time>±hh |
Time zones in ISO 8601 are represented as local time, 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. It is usually preferable to indicate a time zone 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 "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".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 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 depending on context.
Time offsets from UTC
The UTC offset is appended to the time in the same way that 'Z' was above, in the form ±:, ±, or ±.Negative UTC offsets describe a time zone west of, where the civil time is behind than UTC so the zone designator will look like "−03:00","−0300", or "−03".
Positive UTC offsets describe a time zone east of, where the civil time is ahead than UTC so the zone designator will look like "+02:00","+0200", or "+02".
Examples
- "−05:00" for New York on standard time
- "−04:00" for New York on daylight saving time
- "+00:00" for London
- "+02:00" for Cairo
- "+05:30" for Mumbai
- "+14:00" for Kiribati
To represent a negative offset, ISO 8601 specifies using either a hyphen-minus or a minus sign character. If the interchange character set is limited and does not have a minus sign character, then the hyphen-minus should be used. ASCII does not have a minus sign, so its hyphen-minus character would be used. If the character set has a minus sign, then that character should be used. Unicode has a minus sign, and its character code is U+2212 ; 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 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 − 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,. It is permitted to omit the "T" character by mutual agreement as in.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 accuracy format.
Durations
Durations define the amount of intervening time in a time interval and are represented by the format PYMDTHMS or PW as shown to the right. 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 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.
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. 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 or a 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.
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.
Although the standard describes a duration as part of time intervals, which are discussed in the next section, the duration format 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. The two time points 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
For
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 means an unbounded number of repetitions. If the interval specifies the start, then this is the start of the repeating interval. If the interval specifies the end but not the start, 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, 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.Usage
On the Internet, the World Wide Web Consortium uses 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.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 standards for TV, digital radio, etc. use several forms to describe points in time and durations. The ID3 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
RFC 3339 defines a profile of ISO 8601 for use in Internet protocols and 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
| Australia | AS ISO 8601-2007 |
| Austria | ÖNORM ISO 8601 |
| Belgium | NBN EN 28601 |
| Brazil | NBR 5892:2019 |
| Canada | CAN/CSA-Z234.4-89 |
| Colombia | NTC 1034:2014 |
| China | GB/T 7408-2005 |
| Czech Republic | ČSN ISO 8601 |
| Denmark | DS/ISO 8601:2005 |
| Estonia | EVS 8:2008; EVS-ISO 8601:2011 |
| European Norm | EN ISO 8601, EN 28601:1992 |
| Finland | SFS-EN 28601 |
| France | NF Z69-200; NF EN 28601:1993-06-01 |
| Germany | DIN ISO 8601:2006-09 ; related: DIN 5008:2011-04 |
| Greece | ELOT EN 28601 |
| Hungary | MSZ ISO 8601:2003 |
| Iceland | IST EN 28601:1992 |
| India | IS 7900:2001 |
| Ireland | IS/EN 28601:1993 |
| Italy | UNI EN 28601 |
| Japan | JIS X 0301:2002 |
| Korea, Republic of | KS X ISO 8601 |
| Lithuania | LST ISO 8601:2006 |
| Luxembourg | ITM-EN 28601 |
| Netherlands | NEN ISO 8601, NEN EN 28601 , NEN 2772 |
| Norway | NS-ISO 8601 |
| Poland | PN-EN 28601:2002 |
| Portugal | NP EN 28601 |
| Russia | ГОСТ ИСО 8601-2001, ГОСТ 7.64-90 |
| South Africa | SANS 8601:2009 |
| Spain | UNE EN 28601:1995 |
| Sweden | SS-ISO 8601:2011 |
| Switzerland | SN ISO 8601:2005-08 |
| Taiwan | CNS 7648 |
| Thailand | TIS 1111:2535 |
| Turkey | TS ISO 8601 |
| Ukraine | ДСТУ ISO 8601:2010 |
| United Kingdom | BS ISO 8601:2004, BS EN 28601 |
| United States | ANSI INCITS 30-1997 and NIST FIPS PUB 4-2 |
| Vietnam |