International Standard Date and Time Notation, by Markus Kuhn

Note: This is a modified copy of the original document. The original document moved and was unavailable for some time and I often cite it as reference in my writings. Enjoy, David L. Norris

Date
Time of Day
Time Zone

Perspective

Eliminate confusion among international web users by following established standards. Web authors and software engineer who design user interfaces, file formats, and communication protocols should be familiar with ISO 8601.

Brief

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.

Date

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 ISO 8601

easily readable and writeable by software (no 'JAN', 'FEB', ... table necessary)
easily comparable and sortable with a trivial string comparison
language independent
can not be confused with other popular date notations
consistency with the common 24h time notation system, where the larger units (hours) are also written in front of the smaller ones (minutes and seconds)
strings containing a date followed by a time are also easily comparable and sortable (e.g. write "1995-02-04 22:45:00")
the notation is short and has constant length, which makes both keyboard data entry and table layout easier
identical to the Chinese date notation, so the largest cultural group (>25%) on this planet is already familiar with it :-)
date notations with the order "year, month, day" are in addition already widely used e.g. in Japan, Korea, Hungary, Sweden, Finland, Denmark, and a few other countries and people in the U.S. are already used to at least the "month, day" order
a 4-digit year representation avoids overflow problems after 1999-12-31

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.

Alternative Formats

Applications with special requirements can make use of alternate formats provided by ISO-8601. All of these alternatives can easily and automatically be distinguished from each other.

Hyphenation Omission: Hyphens can be omitted if compactness is more important than human readability, for example as in 19950204
Century Omission: Applications where century information is not important may be represented as 95-02-04
Month or Day Omission: If only the year or month is of interest then a format such as 1995 or 1995-02 may be used.
Week Instead of Month: Commercial and industrial applications (delivery times, production plans, etc.), especially in Europe, often require references to 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.; Sometimes this week-day format, 95W05, is useful as a compact code printed on a product which indicates when it has been manufactured.
Week-Weekday instead of Month-MonthDay: 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.
Year-Day Notation: 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

Week Notations

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 (Refering to ANSI C 1992; ISO C 1999 may also apply) 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.]

Leap Year

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.

Time of Day

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 seconds since the start of the minute (00-60). The value 60 for ss appears only in case of an inserted leap second into an atomic time scale like UTC in order to keep it synchronized with a less constant astronomical time scale like UT1. The hour value 24 is only possible when the minute and second values are zero. An example time is 23:59:59 which represents the time one second before midnight.

Compression

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 or 23

Fractional Time

It is also possible to add fractions of a second after a decimal dot, e.g. the time 5.8 ms before midnight can be written as 23:59:59.9942

Midnight

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 1995-02-04 24:00 and 1995-02-05 00:00 refer to exactly the same point in time.

In case an unambiguous representation of time is required, 00:00 is usually the preferred notation for midnight, which is also what most digital clocks display.

A Remark for U.S. Readers

Standard 24h Notation

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. Other languages than English 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.

Old English 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).
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.
It is longer than the normal 24h notation.

The 12h time is simply a relic from the dark ages when Roman numerals were used, the number zero had not yet been invented and analog clocks where 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 recommends now to use the international standard time notation in publications.

A Remark for German Readers

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. 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.

Time Zone

Universal Time Zone

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 Coordinated Universal Time (UTC or CUT), you can append a capital letter Z to a time as in 23:59:59Z.

Z means zero meridian, which goes through Greenwich in London. It is also commonly used in radio communication where it is sounded as Zulu, the word for Z in the International Phonetic Alphabet. Coordinated Universal Time, also called "Zulu Time") was called Greenwich Mean Time (GMT) before 1972. The reference to GMT 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. Primarily, UTC differs from GMT in that GMT observes a daylight adjustments over the course of the year. (i.e. an hour as added or removed every six months to correct for the shifting of the local noon.)

Local Time Zones

Time zones east of the zero meridian can add +hh:mm or +hh to the time to indicate that the used local time zone is hh hours and mm minutes ahead of UTC. Time zones west of the zero meridian, which are behind UTC, can similarly add -hh:mm or -hh to their time notation. For example, Central European Time (CET) is +0100 and U.S./Canadian Eastern Standard Time (EST) is -0500. 12:00Z and 13:00+01:00 and 0700-0500 all indicate the same point in time.

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.

More Information about Time Zones

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@elsie.nci.nih.gov mailing list, which is dedicated to discussions about time zones, please send a short message to tz-request@elsie.nci.nih.gov. You can read previous discussion there in the tz archive.