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ISO 8601 Dates: What They Are and How to Use Them
Contents 1. What They Are 2. In Programs 3. In Output 4. On Personal Computers
5. Fix Data/Programs A1. Important Dates A2. Leap Years A3. A Y2K Parable  

ISO 8601: The Right Format for Dates


Wanted: An Unambiguous Numerical Format for Dates
  The right format for dates in data -- and in the real world -- is one that is totally unambiguous. When you see a written date, is it clear what date that really is?

For example, in the United States, June 23, 1998 is often written as 06/23/98. In Europe, it's 23.06.98 or perhaps 98/06/23. We humans can tell that 06/23/98 and 98/06/23 are the same -- 23 has to be a day of the month because there are only 12 months. (But humans in the US might not recognize 23.06.98 as a date at all.)

Other dates do not fare as well -- the 12th or earlier day of any month, for instance, or any date after January 2, 2001 -- 01/02/01. Or is that February 1, 1001? (We're good for the which one is the year question in the year 2000, because 00 has to be a year. January 1, 2001 is totally OK; its 01/01/01 in all three forms.)

Writing a four-digit year will at least tell you which one is the year. But were still left the other problem, which is more important globally: Is 02/01/2001 February 1, 2001 or January 2, 2001? What to do?

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ISO Standard 8601 for Dates (and Times)
  All the way back in 1988, the International Standards Organization (ISO) decided, for obvious reasons, that these country-specific all-numeric date formats had to go. The ISO standard date format is defined in the ISO 8601:

ISO 8601 defines acceptable formats for times as well; it's available in PDF form at:
(For additional links, see J R Stockton's Date and Time Formats page's ISO standard 8601 section,

This ISO 8601 format for dates has a number of good features.

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Written Dates: Unambiguous But Not Useful for Data
  In written text, the traditional written forms of a date are also unambiguous, because a four-digit year is used and the month name is written as a word rather then a number. In English, we have the American form, June 23, 1998, and the British form, 23 June 1998. Regardless of the order, you can tell that the four-digit item is the year, the word is the month, and the two-digit item is the day. However, these literate forms should be avoided in any data processing use, because data is generally language-free and because dates in these formats cannot be sorted without conversion.

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Unambiguous Around the World
  Being unambiguous around the world remains the absolute requirement. In the financial world, for instance, an ambiguous date on a transaction could mean a difference of a significant amount of money, if interest calculations are involved. For this reason, the only acceptable date formats should be either the full literate forms, with a four-digit year of course, such as the American English "June 23, 1998" or the British English "23 June 1998", or the ISO 8601 form, "1998-06-23". After you read the links below, you will understand why there can be no other date formats tolerated in this day of global communication. And, for other than literary purposes, such as for finance or data processing, the all-numeric ISO form should be the only form used.

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For a Good Time
  The next time you fill out a form for anyone, whether online or on paper, see how many times you are asked for a date with a something that looks like this:
_ _ _ _ _ _

If you find it, the form has a year 2000 problem, and it's reasonable to assume that the software that process the data entered on the form does too. You might want to let them know.

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For More Information about Date Formats
  See: And for step-by-step instructions on how to use ISODATES in your personal computer operating systems and in the major programming languages and statistical software packages, see

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This page last updated 2001-12-21. Please send comments and reports of broken links to the author: Roger Deschner

Using ISO 8601 Dates Previous:  Contents Next:  2. In Programs

2001-12-21  ACCC Documentation
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