by Markus Kuhn + +
International Standard +ISO 8601 specifies numeric representations of date and time. This +standard notation helps to avoid confusion in international +communication caused by the many different national notations and +increases the portability of computer user interfaces. In addition, +these formats have several important advantages for computer usage +compared to other traditional date and time notations. The time +notation described here is already the de-facto standard in almost all +countries and the date notation is becoming increasingly popular. + +
Especially authors of Web pages and software engineers who +design user interfaces, file formats, and communication protocols +should be familiar with ISO 8601. + +
Contents: Date, Time of Day, +Time Zone. + +
The international standard date notation is + +
+ +YYYY-MM-DD
where YYYY is the year in the usual Gregorian calendar, MM is the +month of the year between 01 (January) and 12 (December), and DD is +the day of the month between 01 and 31. + +
For example, the fourth day of February in the year 1995 is written +in the standard notation as + +
+ +1995-02-04
Other commonly used notations are e.g. 2/4/95, 4/2/95, 95/2/4, +4.2.1995, 04-FEB-1995, 4-February-1995, and many more. Especially the +first two examples are dangerous, because as both are used quite often +in the U.S. and in Great Britain and both can not be distinguished, it +is unclear whether 2/4/95 means 1995-04-02 or 1995-02-04. The date +notation 2/4/5 has at least six reasonable interpretations (assuming +that only the twentieth and twenty-first century are reasonable +candidates in our life time). + +
Advantages of the ISO 8601 standard date notation compared to other +commonly used variants: + +
As dates will look a little bit strange anyway starting with +2000-01-01 (e.g. like 1/1/0), it has been suggested that the year 2000 +is an excellent opportunity to change to the standard date notation. + +
ISO 8601 is only specifying numeric notations and does not cover +dates and times where words are used in the representation. It is not +intended as a replacement for language-dependent worded date notations +such as "24. Dezember 2001" (German) or "February 4, 1995" (US +English). ISO 8601 should however be used to replace notations such as +"2/4/95" and "9.30 p.m.". + +
Apart from the recommended primary standard notation +YYYY-MM-DD, ISO 8601 also specifies a number of +alternative formats for use in applications with special requirements. +All of these alternatives can easily and automatically be +distinguished from each other: + +
The hyphens can be omitted if compactness of the representation is +more important than human readability, for example as in + +
+ +19950204
For situations where information about the century is really not +required, a 2-digit year representation is available: + +
+ +95-02-04 or +950204
If only the month or even only the year is of interest: + +
+ +1995-02 or +1995
In commercial and industrial applications (delivery times, +production plans, etc.), especially in Europe, it is often required to +refer to a week of a year. Week 01 of a year is per definition the +first week that has the Thursday in this year, which is equivalent to +the week that contains the fourth day of January. In other words, the +first week of a new year is the week that has the majority of its +days in the new year. Week 01 might also contain days from the +previous year and the week before week 01 of a year is the last week +(52 or 53) of the previous year even if it contains days from the new +year. A week starts with Monday (day 1) and ends with Sunday (day 7). +For example, the first week of the year 1997 lasts from 1996-12-30 to +1997-01-05 and can be written in standard notation as + +
+ +1997-W01 or +1997W01
The week notation can also be extended by a number indicating the +day of the week. For example, the day 1996-12-31, which is the Tuesday +(day 2) of the first week of 1997, can also be written as + +
+ +1997-W01-2 or +1997W012
for applications like industrial planning where many things like +shift rotations are organized per week and knowing the week number and +the day of the week is more handy than knowing the day of the month. + +
An abbreviated version of the year and week number like + +
+ +95W05
is sometimes useful as a compact code printed on a product that +indicates when it has been manufactured. + +
The ISO standard avoids explicitly stating the possible range of +week numbers, but this can easily be deduced from the definition: + +
+ ++ +Theorem: Possible ISO week numbers are in the +range 01 to 53. A year always has a week 52. (There is one historic +exception: the year in which the Gregorian calendar was introduced had +less than 365 days and less than 52 weeks.) + +
Proof: Per definition, the first week of a year is +W01 and consequently days before week W01 belong to the previous year +and so there is no week with lower numbers. Considering the highest +possible week number, the worst case is a leap year like 1976 that +starts with a Thursday, because this keeps the highest possible number +of days of W01 in the previous year, i.e. 3 days. In this case, the +Sunday of W52 of the worst case year is day number 4+51*7=361 and +361-366=5 days of W53 belong still to this year, which guarantees that +in the worst case year day 4 (Thursday) of W53 is not yet in the next +year, so a week number 53 is possible. For example, the 53 weeks of +the worst case year 1976 started with 1975-12-29 = 1976-W01-1 and +ended with 1977-01-02 = 1976-W53-7. On the other hand, considering the +lowest number of the last week of a year, the worst case is a non-leap +year like 1999 that starts with a Friday, which ensures that the first +three days of the year belong to the last week of the previous year. +In this case, the Sunday of week 52 would be day number 3+52*7=367, +i.e. only the last 367-365=2 days of the W52 reach into the next year +and consequently, even a worst case year like 1999 has a week W52 +including the days 1999-12-27 to 2000-01-02. q.e.d. + +
[Unfortunately, the current version of the C programming language
+standard provides in the strftime() function no means to
+generate the ISO 8601 week notation. A required extension would be
+four new formatting codes: for the year of the week to which the
+specified day belongs (both 2-digit and 4-digit), for the number of
+the week between 01 and 53, and for the day of the week between 1
+(Monday) and 7 (Sunday). Another trivial mistake in the description of
+strftime() in the C standard is that the range of seconds
+goes from 00 to 61, because at one time only one single leap second 60
+can be inserted into UTC and consequently there will never be a leap
+second 61. Contribution N764 to the
+ISO C committee
+suggests to fix some of this in the next revision of the ISO C
+standard. The author of this text has also developed a proposal for a
+modernised clock and calendar API for C, which
+provides full proper treatment of leap seconds and timezones and fixes
+numerous other problems in the current C timing library functions. It
+also serves as an excellent model for those who want to design clock
+library functions for other programming languages.]
+
+
Both day and year are useful units of structuring time, because the +position of the sun on the sky, which influences our lives, is +described by them. However the 12 months of a year are of some obscure +mystic origin and have no real purpose today except that people are +used to having them (they do not even describe the current position of +the moon). In some applications, a date notation is preferred that +uses only the year and the day of the year between 001 and 365 (366 in +leap years). The standard notation for this variant representing +the day 1995-02-04 (that is day 035 of the year 1995) is + +
+ +1995-035 or +1995035
Leap years are years with an additional day YYYY-02-29, where the +year number is a multiple of four with the following exception: If a +year is a multiple of 100, then it is only a leap year if it is also a +multiple of 400. For example, 1900 was not a leap year, but 2000 is one. + +
The international standard notation for the time of day is + +
+ +hh:mm:ss
where hh is the number of complete hours that have passed since +midnight (00-24), mm is the number of complete minutes that have +passed since the start of the hour (00-59), and ss is the number of +complete seconds since the start of the minute (00-59). If the hour +value is 24, then the minute and second values must be zero. [Although +ISO 8601 does not mention this, the value 60 for ss might sometimes be +needed during an inserted leap second in an +atomic time scale like Coordinated Universal Time (UTC). A single leap +second 23:59:60 is inserted into the UTC time scale every few years as +announced by the International Earth +Rotation Service in Paris to keep UTC from wandering away more +than 0.9 s from the less constant astronomical time scale UT1 +that is defined by the actual rotation of the earth.] + + +
An example time is + +
+ +23:59:59
which represents the time one second before midnight. + +
As with the date notation, the separating colons can also be +omitted as in + +
+ +235959
and the precision can be reduced by omitting the seconds or both +the seconds and minutes as in + +
+ +23:59, 2359, or +23
It is also possible to add fractions of a second after a decimal +dot or comma, for instance the time 5.8 ms before midnight can be +written as + +
+ +23:59:59.9942 or +235959.9942
As every day both starts and ends with midnight, the two notations +00:00 and 24:00 are available to +distinguish the two midnights that can be associated with one date. +This means that the following two notations refer to exactly the same +point in time: + +
+ +1995-02-04 24:00 = +1995-02-05 00:00
In case an unambiguous representation of time is required, 00:00 is +usually the preferred notation for midnight and not 24:00. Digital +clocks display 00:00 and not 24:00. + +
ISO 8601 does not specify, whether its notations specify a point in +time or a time period. This means for example that ISO 8601 does not +define whether 09:00 refers to the exact end of the ninth hour of the +day or the period from 09:00 to 09:01 or anything else. The users of +the standard must somehow agree on the exact interpretation of the +time notation if this should be of any concern. + +
If a date and a time are displayed on the same line, then always +write the date in front of the time. If a date and a time value are +stored together in a single data field, then ISO 8601 suggests that +they should be separated by a latin capital letter T, as in +19951231T235959. + +
A remark for readers from the U.S.: + +
+ +The 24h time notation specified here has already been +the de-facto standard all over the world in written language for +decades. The only exception are some English speaking countries, where +still notations with hours between 1 and 12 and additions like "a.m." +and "p.m." are in wide use. The common 24h international standard +notation starts to get widely used now even in England. Most other +languages don't even have abbreviations like "a.m." and "p.m." and the +12h notation is certainly hardly ever used on Continental Europe to +write or display a time. Even in the U.S., the military and computer +programmers have been using the 24h notation for a long time. + +
The old English 12h notation has many disadvantages like: + +
+
+ +- It is longer than the normal 24h notation. +
- It takes somewhat more time for humans to compare two times + in 12h notation. +
- It is not clear, how 00:00, 12:00 and 24:00 are represented. + Even encyclopedias and style manuals contain contradicting + descriptions and a common quick fix seems to be to avoid + "12:00 a.m./p.m." altogether and write "noon", "midnight", or + "12:01 a.m./p.m." instead, although the word "midnight" still + does not distinguish between 00:00 and 24:00. +
- It makes people often believe that the next day starts at the + overflow from "12:59 a.m." to "1:00 a.m.", which is a common + problem not only when people try to program the timer of VCRs + shortly after midnight. +
- It is not easily comparable with a string compare operation. +
- It is not immediately clear for the unaware, whether the + time between "12:00 a.m./p.m." and "1:00 a.m./p.m." starts + at 00:00 or at 12:00, i.e. the English 12h notation is more + difficult to understand. +
Please consider the 12h time to be a relic from the dark ages when +Roman numerals were used, the number zero had not yet been invented +and analog clocks were the only known form of displaying a +time. Please avoid using it today, especially in technical +applications! Even in the U.S., the widely respected Chicago +Manual of Style now recommends using the international +standard time notation in publications. + +
A remark for readers from German speaking countries: + +
+ +In May 1996, the German standard DIN 5008, which +specifies typographical rules for German texts written on typewriters, +has been updated. The old German numeric date notations DD.MM.YYYY and +DD.MM.YY have been replaced by the ISO date notations YYYY-MM-DD and +YY-MM-DD. Similarly, the old German time notations hh.mm and hh.mm.ss +have been replaced by the ISO notations hh:mm and hh:mm:ss. Those new +notations are now also mentioned in the latest edition of the +Duden. The German alphanumeric date notation continues to +be for example "3. August 1994" or "3. Aug. 1994". The corresponding +Austrian standard has already used the ISO 8601 date and time +notations before. + +
ISO 8601 has been adopted as European Standard EN 28601 and is +therefore now a valid standard in all EU countries and all conflicting +national standards have been changed accordingly. +
Without any further additions, a date and time as written above is +assumed to be in some local time zone. In order to indicate that a +time is measured in Universal Time (UTC), you can append a capital +letter Z to a time as in + +
+ +23:59:59Z or 2359Z +
[The Z stands for the "zero meridian", which goes through Greenwich +in London, and it is also commonly used in radio communication where +it is pronounced "Zulu" (the word for Z in the international radio +alphabet). Universal +Time (sometimes also called "Zulu Time") was called Greenwich Mean +Time (GMT) before 1972, however this term should no longer be +used. Since the introduction of an international atomic time scale, +almost all existing civil time zones are now related to UTC, which is +slightly different from the old and now unused GMT.] + +
The strings + +
+ ++hh:mm, +hhmm, or ++hh
can be added to the time to indicate that the used local time zone +is hh hours and mm minutes ahead of UTC. For time zones west of the +zero meridian, which are behind UTC, the notation + +
+ +-hh:mm, -hhmm, or +-hh
is used instead. For example, Central European Time (CET) is +0100 +and U.S./Canadian Eastern Standard Time (EST) is -0500. The following +strings all indicate the same point of time: + +
+ +12:00Z = 13:00+01:00 += 0700-0500
There exists no international standard that specifies +abbreviations for civil time zones like CET, EST, etc. and sometimes +the same abbreviation is even used for two very different time zones. +In addition, politicians enjoy modifying the rules for civil time +zones, especially for daylight saving times, every few years, so the +only really reliable way of describing a local time zone is to specify +numerically the difference of local time to UTC. Better use directly +UTC as your only time zone where this is possible and then you do not +have to worry about time zones and daylight saving time changes at +all. + +
Arthur David Olson and +others maintain a database of all current and +many historic time zone changes and daylight saving time +algorithms. It is available via ftp from elsie.nci.nih.gov in the +tzcode* and tzdata* files. Most Unix time +zone handling implementations are based on this package. If you want +to join the tz mailing list, which is dedicated to +discussions about time zones and this software, please send a request +for subscription to tz-request@elsie.nci.nih.gov. You can read previous discussion +there in the tz +archive. + +
Some other interesting sources of information about date and time +on the Internet are for example the Glossary of +Frequency and Timing Terms and the FAQ +provided by NIST, +the Yahoo +Science:Measurements and Units:Time link collection, the U.S. Naval Observatory Server, +the International Earth Rotation +Service (IERS) (for time gurus only!), the University of Delaware NTP Time +Server, the time and calendar section of the USENET sci.astro +FAQ, and the Calendar FAQ. + +
This was a brief overview of the ISO 8601 standard, which covers +only the most useful notations and includes some additional related +information. The full standard defines in addition a number of more +exotic notations including some for periods of time. The ISO 8601:1988 document is +fortunately now also available online, or you +can order a paper copy from + +
+ ++International Organization +for Standardization
+Case postale 56
+1, rue de Varembé
+CH-1211 Genève 20
+Switzerland
+
+phone: +41 22 749 01 11
+fax: +41 22 733 34 30
+email: sales@isocs.iso.ch +
A more detailed online summary of ISO 8601 than this one is the +text ISO 8601:1988 Date/Time Representations available +from +ftp.informatik.uni-erlangen.de/pub/doc/ISO/ISO8601.ps.Z +(PostScript, 16 kb, 5 pages) written by Gary Houston, now also +available in HTML. Ian Galpin (G1SMD) proposes to use ISO 8601 as a Common Date-Time +Standard for Amateur Radio. Steve Adams has written another web +page about the ISO date format that is partially based on this +text. + +
ISO TC 154 decided in 1996 to revise ISO 8601. Louis Visser is coordinating +this project. If you want to contribute to this work, you should +contact your national ISO member +organization. + +
I wish to thank Edward +M. Reingold for developing the fine GNU Emacs calendar functions, +as well as Rich Wales, +Mark Brader, Paul Eggert, and others +in the comp.std.internat, comp.protocols.time.ntp, and +sci.astro USENET discussion groups for +valuable comments about this text. Further comments and hyperlinks to +this page are very welcome. + +
Some journalists recently got interested in the international date +and time format and reported about it. Examples include: +
If you are a journalist and need information on this or related +topics, please feel free to contact me. + +
You might also be interested in the International +Standard Paper Sizes Web page. + +
+Markus Kuhn <Markus.Kuhn@cl.cam.ac.uk>
+
created 1995 -- last modified 2000-01-24 --
+http://www.cl.cam.ac.uk/~mgk25/iso-time.html
+
+