ISO 8601

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Example combined date/time in UTC:

2009-05-17T13:17Z

Example separate date and time in UTC:

2009-05-17 13:17Z

Example date only:

2009-05-17

Example date with week number:

2009-W20-7

ISO 8601 is an international standard for date and time representations issued by the International Organization for Standardization (ISO). Specifically, this standard defines: "Data elements and interchange formats — Information interchange — Representation of dates and times". The signature feature of the ISO 8601 format for date and time is that the information is ordered from the most to the least significant or, in plain terms, from the largest (the year) to the smallest (the second).

Contents 1 History of the standard 2 General principles 3 Dates 3.1 Years 3.2 Calendar date 3.3 Week dates 3.4 Ordinal dates 4 Times 4.1 Time zones 4.1.1 UTC 4.1.2 Other time zones 4.1.3 Time zone conversions 5 Combined representations 6 Duration 7 Time interval 7.1 Repeating intervals 8 Truncated representations 9 Notes 10 See also 11 External links 11.1 Implementations

[edit] History of the standard

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 2015, ISO 2711, ISO 3307, and ISO 4031^[1]. It has since then been superseded by a second edition in 2000 and finally the current third edition, ISO 8601:2004, published 2004-12-03.

ISO 2014 was the standard that originally introduced the big-endian all-numeric date notation [YYYY]-[MM]-[DD]. 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/ITC 154.

[edit] General principles

Date and time values are organized from most to least significant. Each value (e.g. year, month, day, time) has a fixed number of digits which must be padded with leading zeros. For instance, the notation "4:30 a.m." would be written 04:30. As a result, for each of the several formats of dates and times lexicographical order corresponds to chronological order, except for negative years.

Representation can be done in one of two formats: A basic format with a minimal number of characters, or an extended format with separators to enhance human readability.^[2] The standard permits a hyphen separator between date elements and a colon between hours, minutes, and seconds. For example, "2006-01-06" may be written "20060106" without ambiguity. The extended formats are preferred over basic formats because some basic formats are ambiguous.

Any number of fields may be dropped from any of the date and time formats, but the least significant fields must be dropped first. For example, "2004-05" is a valid ISO 8601 date, which indicates the 5th month of the year 2004. This date will never represent the 5th day of some unknown month in 2004.

Finally, the standard supports the addition of a decimal fraction to the smallest time unit, where higher precision is needed.

[edit] Dates

<< < May 2009 > >>
Wk	Mo	Tu	We	Th	Fr	Sa	Su
(18)	27	28	29	30	1	2	3
(19)	4	5	6	7	8	9	10
(20)	11	12	13	14	15	16	17
(21)	18	19	20	21	22	23	24
(22)	25	26	27	28	29	30	31

The standard uses the leap rule and month sizes of the Gregorian calendar, which is already the de facto standard of international trade.

YYYY
±YYYYY

[edit] Years

ISO 8601 prescribes a four-digit year to avoid the year 2000 problem.

To represent years before 0 or after 9999, the standard also permits expansion of the year by agreement between sender and receiver^[3]. An expanded representation may have (an agreed number of) extra year digits, and is always prefixed with a sign (+ or −), with the convention that 0 is positive.

For reference purposes, ISO 8601 assigns the number 1875 to the year in which the Convention du Mètre was signed in Paris – a choice that has been criticized^{[citation needed]} for not being reproducible without another calendar.

[edit] Calendar date

YYYY-MM-DD	or	YYYYMMDD
YYYY-MM	but not	YYYYMM

Calendar dates are the form familiar to most people. As represented in the box to the right, [YYYY] indicates a year with century. [MM] indicates the month of the year, 01 through 12. [DD] indicates the day of that month, from 01 through 31. For example, "5th of April 1981" may be represented as "1981-04-05" in the extended format, or "19810405" in the basic format.

The standard allows for dates to be written with less precision. For example, one may write "1981-04" to mean "1981 April", and one may simply write "1981" to refer to that year. The standard allows both YYYYMMDD and YYYY-MM-DD, but if the day is omitted, only YYYY-MM is allowed. By disallowing dates of the form YYYYMM, the standard avoids confusion with the truncated representation YYMMDD (still often used).

[edit] Week dates

YYYY-Www	or	YYYYWww
YYYY-Www-D	or	YYYYWwwD

Main article: ISO week date

A week date specifies a so-called ISO year in the format [YYYY], a week number in the format [WW] prefixed by the letter 'W', and the weekday number, a digit [D] from 1 through 7, beginning with Monday and ending with Sunday. This form is popular in manufacturing.

Mutually equivalent definitions for week 01 are:

• the week with the year's first Thursday in it
• the week with 4 January in it
• the first week with the majority (four or more) of its days in the starting year
• 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.

The week number can be described by counting the Thursdays: week 12 contains the 12th Thursday of the year.

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

Examples:

• 2008-12-29 is written "2009-W01-1"
• 2010-01-03 is written "2009-W53-7"

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 year is needed, while correspondence of lexicographical order and chronological order is preserved.

[edit] Ordinal dates

YYYY-DDD	or	YYYYDDD

Ordinal dates are a simple form for times 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, "1981-04-05" is also "1981-095".

This format has particular use for simple hardware systems that have need of a date system, but where including full calendar calculation software may be a significant nuisance. This system is frequently, though incorrectly, referred to as the Julian Date.

[edit] Times

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

ISO 8601 uses the 24-hour clock system. The basic format is [hh][mm][ss] and the extended format is [hh]:[mm]:[ss]. [hh] refers to a zero-padded hour between 00 and 24, where 24 is only used to notate the midnight at the end of a calendar date. [mm] refers to a minute between 00 and 59. [ss] refers to a second between 00 and 59 (or 60 in the exceptional case of an added leap second). So a time might appear as "13:47:30" or "134730".

It is also acceptable to omit elements to reduce precision. [hh]:[mm], [hh][mm] and [hh] are all used.

Decimal fractions may also be used with any of the three time elements. These are indicated by using the decimal point (either a comma (which is preferred ISO 31) or dot). A fraction may only refer to the most precise component of a time representation. To denote "14 hours, 30 and one half minutes", do not include a seconds figure. Represent it as "14:30,5" or "1430,5".

Midnight 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 the day, and is the most frequently used one. 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 representations below).

[edit] Time zones

<time>Z
<time>±hh:mm	or	<time>±hhmm
<time>±hh

If no time zone information is given with a time, the time zone is assumed to be in some conventional local time zone. While it may be safe to assume a local zone when used between two people in the same area, it is ambiguous when used in communication between multiple time zones. It is usually preferable to indicate a time zone (zone designator) using the standard’s notation.

[edit] UTC

If the time is in UTC, add a 'Z' directly after the time, without a space; 'Z' is the zone designator for zero 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'.

[edit] Other time zones

The zone designator for other time zones is specified by the offset from UTC 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 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. Note that the zone designator is the actual offset from UTC and does not include any information on daylight saving time. The zone designator or UTC offset for a user in Chicago 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 zone letters are not used with the exception of Z.

[edit] Time zone conversions

Conversion between time zones obeys the relationship

"time in zone A" - "UTC offset for zone A" = "time in zone B" - "UTC offset for zone B",

in which each side of the equation is equivalent to UTC.

Using the term "UTC offset" rather than zone designator as the standard does, the conversion equation can be rearranged to

"time in zone B" = "time in zone A" - "UTC offset for zone A" + "UTC offset for zone B".

For example, what time is it in Los Angeles when the New York Stock Exchange opens?

time in zone PST = 09:30 - (-05:00) + (-08:00) = 06:30

In Delhi, the New York Stock Exchange opens at

time in India zone = 09:30 - (-05:00) + (+5:30) = 20:00

[edit] Combined representations

<date>T<time>	<date>T<time>Z

Combining date and time representations into a single representation is quite simple. It is in the format of <date>T<time> where the time designator [T] is used to show the start of the time elements of the representation. The date and time are any proper representations of date and time specified by the standard. A common use could be [YYYY]-[MM]-[DD]T[hh]:[mm]. "2007-04-05T14:30" is an example using this format, or "20071023T1200Z" which uses the format [YYYY][MM][DD]T[hh][mm]Z.

The date and time representations may appear in proximity to each other, often separated by a space or sometimes by other characters. In these cases they occupy two separate fields in a data system, rather than a single combined representation. This is usually done for human readability. Unlike the previous examples, "2007-04-05 14:30" is considered two separate, but acceptable, representations—one for date and the other for time.

[edit] Duration

PnnYnnMnnDTnnHnnMnnS
PnnYnnWnnDTnnHnnMnnS
P<date>T<time>

Durations are represented by the format P[n]Y[n]M[n]DT[n]H[n]M[n]S (or P[n]Y[n]W[n]DT[n]H[n]M[n]S to use the week format). In this representation replace [n] with the appropriate number for the element that follows it (leading zeros are optional but may clarify ambiguous durations). The capital letters ('P', 'Y', 'M', 'W', 'D', 'T', 'H', 'M' and 'S') are used as they are and not replaced. Thus "P3Y6M4DT12H30M0S" defines "a period of three years, six months, four days, twelve hours, thirty minutes, and zero seconds". Elements may be omitted if their value is zero. To resolve ambiguity, "P1M" is one month and "PT1M" is one minute. The smallest value used may also have a decimal fraction, as in "P0,5Y" to indicate half a year.

Sometimes ISO 8601 is called the "metric date format", but the chosen abbreviations in durations do not match common metric symbols (a, mon, d, h, min, s).

Alternately, a format more based on the combined representation may be used: P[YYYY]-[MM]-[DD]T[hh]:[mm]:[ss]. To represent the same duration as above in this format, use "P0003-06-04T12:30:00".

[edit] Time interval

<start>/<end>
<start>/<duration>
<duration>/<end>
<duration>

Time intervals specify an amount of time. They may be expressed in four ways:

1. Start and end, such as "2007-03-01T13:00:00Z/2008-05-11T15:30:00Z"
2. Start and duration, such as "2007-03-01T13:00:00Z/P1Y2M10DT2H30M"
3. Duration and end, such as "P1Y2M10DT2H30M/2008-05-11T15:30:00Z"
4. Duration only, such as "P1Y2M10DT2H30M"

Of these, the first three require two separate values separated by an interval designator which is usually a solidus or forward slash "/". In certain applications, a double hyphen (--) can be used as a separator instead of a solidus (see Section 4.4.2 of the standard).

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, or the beginning and end dates of a monthly billing period as 2008-02-15/03-14.

If greater precision is desirable to represent the time interval, then more time elements can be added to the representation. An observation period that has a duration of approximately three days, for example, can be succinctly shown as 2007-11-13/15. If the exact start and end of the observation period need to be shown for clarity or for measurement and recording purposes, the same time interval representation could be expanded to 2007-11-13T00:00/15T24:00.

[edit] Repeating intervals

Rnn/<interval>
R/<interval>

Repeating intervals 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] means an unbounded number of repetitions. So, to repeat the interval of "P1Y2M10DT2H30M" five times starting at "2008-03-01T13:00:00Z", use "R5/2008-03-01T13:00:00Z/P1Y2M10DT2H30M".

[edit] 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.

[edit] Notes

[edit] See also

• Date and time notation by country
• ISO 8601 usage
• Template:ISO8601
• RFC 3339, Date and Time on the Internet, 2002-July

[edit] External links

• ISO 8601:2004(E) from ISO (zip-pdf, 228KB)
• Summary of 8601 by ISO
• Technical Committee ISO/TC 154, Processes, data elements and documents in commerce, industry and administration.
• ISO 8601 summary by Markus Kuhn
• The Mathematics of the ISO 8601 Calendar
• Current date in week date and ordinal date format
• ISO 8601 at the Open Directory Project
• W3C Specification about UTC Date and Time and IETF Internet standard RFC 3339, based on ISO 8601

[edit] Implementations

• Converting and implementing
• An implementation in Java (Joda-Time)
• Setting Your Personal Computer for ISO 8601br:ISO 8601

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