This is an essay. It contains the advice or opinions of one or more Wikipedia contributors. This page is not an encyclopedia article, nor is it one of Wikipedia's policies or guidelines, as it has not been thoroughly vetted by the community. Some essays represent widespread norms; others only represent minority viewpoints.

This page in a nutshell: Dates (individual days of a given month of a four digit year, and four digit years) may be formatted in the same way as they would under ISO 8601, but Wikipedia does not claim that they follow ISO 8601

ISO 8601 is an international standard for communicating certain information, in particular between computer systems. The information comprises certain units of time which would normally be called "dates and times" from millennia (and with extensions larger units) down to seconds and decimal fractions thereof (for example 12:34 on 10 April 1962), and other time-like entities which we need not concern ourselves with here.

For purposes of style the English Wikipedia permits a number of representations of dates which are, or are partially, consistent with the representations of ISO 8601 - however these are not to be considered ISO 8601 dates, although by happy accident they may be consistent in other respects, there is no guarantee, or even attempt to make sure they are.

• In 1952 Queen Elizabeth II....

Constant in style and meaning.

• In 1066 Harold Hardrada...

Consistent in style but not valid in substance - ISO dates cannot be used before 1582 without agreement between "the exchanging parties" - and also need to be in the Gregorian calendar, proleptic if necessary.

• In 986 King Aethelstan ....

Not even consistent with style ISO would have "0986" - 986 would represent the decade 9860-9869.

• In 4 BC King Herod ....

ISO would have "-0004".

• More commonly thought of as "ISO" dates such as 1999-12-04 are seen in citations, quotes and table. Here again while superficially the same as an ISO 8601 date, if for example we specify the time zone in text "1999-12-04 (CET)" it ceases to become valid - moreover there is still no guarantee that even relatively late dates are Gregorian in certain countries

While, clearly, there is nothing to stop quotation of any date, whether ISO valid, ISO style and invalid, or just plain wrong - in non-quoted text - only the YYYY (without leading 0s) and YYYY-MM-DD formats are explicitly permitted by the MoS. For example, 1999-10 for October 1999 is not cromulent with the MoS, and indeed use of numbers to represent months, other than in YYYY-MM-DD is frowned upon. Similarly unexplained use of the finer grained entities than days (hour, minutes, seconds), use of "2" for the third millennium (-1 year) or 198 for the 1980s, or of the recurring periods would be considered obscure at best, and just wrong at worst. We do not use a solidus to represent periods, nor do we use ISO style formats for weeks of the year or days of the year.

"ISO 8601 does not specify the exact meaning of its representations."^[1]

• Gregorian calendar
• ISO 8601
• WP:MOSNUM
• Wikipedia:Date formattings