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 is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988 with updates in 1991, 2000, 2004 and 2019. 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 (and potentially proleptic Gregorian) calendar, of times based on the 24-hour timekeeping system (with optional UTC offset), of time intervals, 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 (e.g., names of years in the Chinese calendar) or that do not use characters (e.g., images, sounds).
In representations for interchange, dates and times are arranged so the largest temporal term (the year) 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 (such as "-", ":", "T", "W", and "Z") 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 It has been superseded by a second edition "ISO 8601:2000" in 2000, by a third edition "ISO 8601:2004" published on 1 December 2004, and withdrawn and revised by "ISO 8601-1:2019" and "ISO 8601-2:2019" on 25 February 2019.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ISO 2014, though superseded, is the standard that originally introduced the all-numeric date notation in most-to-least-significant order .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 (EDTF).
Dates.
The standard uses the Gregorian calendar, 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) was signed in 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, 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 (because at the switchover date, the dates would not be consecutive).
Years.
ISO 8601 prescribes, as a minimum, a four-digit year [YYYY] 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 before 1583 are not automatically allowed by the standard. Instead "values in the range [0000] through [1582] 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 [±YYYYY] 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 (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. [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, "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 from 1980 to 1989 inclusive, 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 [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 formats as shown in the adjacent box. [YYYY] indicates the "ISO week-numbering year" which is slightly different from the traditional Gregorian 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.
There are several mutually equivalent and compatible descriptions of week 01:
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:
Ordinal dates.
An ordinal date 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 years). 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, 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 system. As of ISO 8601-1:2019, the "basic format" is T[hh][mm][ss] and the "extended format" is T[hh]:[mm]:[ss]. Earlier versions omitted the T (representing time) in both formats.
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:
As of ISO 8601-1:2019 "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, either a comma or a dot (following ISO 80000-1 according to ISO 8601:1-2019, which does not stipulate a preference except within International Standards, but with a preference for a comma 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 of a decimal fraction is 3, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".
Time zone designators.
Time zones 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. 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 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 ±[hh]:[mm], ±[hh][mm], or ±[hh].
Negative UTC offsets describe a time zone west of , where the civil time is behind (or earlier) 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 (or later) than UTC so the zone designator will look like "+02:00","+0200", or "+02".
Examples
See List of UTC time offsets for other UTC offsets.
To represent a negative offset, ISO 8601 specifies using a minus sign. 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 (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 has a minus sign, and its character code is U+2212 (2212 hexadecimal); the HTML character entity invocation is codice_1.
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 − (−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[n]Y[n]M[n]DT[n]H[n]M[n]S or P[n]W as shown on the aside. In these representations, the [n] is replaced by the value for each of the date and time elements that follow the [n]. 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.
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 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 (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:
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 filenames; in common operating systems, a solidus is a reserved character and is not allowed in a filename.
For <start>/<end> 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[n]/" to the beginning of an interval expression, where "R" is used as the letter itself and [n] is replaced by the number of repetitions. Leaving out the value for [n] or specifying a value of -1, means an unbounded number of repetitions. A value of 0 for [n] 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 codice_2 for century. All other formats have one leading codice_2 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.
Usage.
On the Internet, the World Wide Web Consortium (W3C) 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. 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 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.
External links.
Implementation overview