YEAR 2000: ISO - INTERNATIONAL STANDARD DATE

By Ziep Vuong  ([email protected])  

The lack of widely accepted Date standards was one of the main factors that lead to the current Y2K problem. There is confusion in interpreting dates because no standard representation of dates has been internationally accepted and implemented. Take for example, the fourth day of February in the year 1995. There are at least six commonly used notations to this date: 2/4/95, 4/2/95, 95/2/4, 4.2.1995, 04-FEB-1995, 4-February-1995. The first two examples are used 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.

Attempts have been made since the mid 1970s to set date standards, and even as recently as the late 1980's the ISO-8601 standard advocated the YYYYMMDD format for date storage. But the use of these standards was never globally mandated and they never achieved critical mass in commercial acceptance. The current Y2K problem has shown that an internationally accepted date standard is needed and the ISO 8601 date notation is becoming increasingly popular. 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. As we are approaching the new millenium, the year 2000 is an excellent opportunity to adopt the ISO standard.

A SUMMARY OF THE INTERNATIONAL STANDARD DATE AND TIME NOTATION

Following is a brief overview of the ISO 8601 standard, which covers the most useful notations as presented by Markus Kuhn (https://www.cl.cam.ac.uk/~mgk25/iso-time.html).

Formats ISO 8601 describes various date-time formats: it defines Extended Format, with punctuation, Basic Format, without punctuation, and it allows some elements to be omitted.

• Date Formats - 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. Advantages of the ISO 8601 standard date notation: · 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.

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 two-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, especially in Europe, it is often required to refer to a week of a year. Week 01 of a year is by 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.

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 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. 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. 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 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 hardly 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. Avoid using the 12h notation, 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 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.

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 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.

Interested people can order a paper copy of ISO 8601 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: [email protected]

To learn more about time you might look for the book "Greenwich Time and the Discovery of Longitude" by Derek Howse published in 1980 by the Oxford University Press. An excellent reference on all matters concerning time is the "Explanatory Supplement to the Astronomical Almanac" edited by P. Kenneth Seidelmann of the U.S. Naval Observatory and published by University Science Books, Mill Valley, CA. There is also a wealth of information on time at the U.S. Naval Observatory's Directorate of Time Web site (https://tycho.usno.navy.mil/time.html).

Baltimore, MD.  Mon, 30 Nov 1998
VACETS Column

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