ISO 8601: Encyclopedia II - ISO 8601 - Dates

ISO 8601 - Dates

The standard uses the Gregorian calendar, already the de facto standard of international trade. The standard acknowledges that other calendars may be used, such as the Julian calendar used by astronomers. It suggests that senders and receivers should explicitly agree when another calendar is used with the standard's notation. Dates are otherwise assumed to be Gregorian. In principle, dates should usually be converted to the proleptic Gregorian calendar to avoid possible confusion.

Years are always integers. Year 0001 corresponds to 1 AD (1 CE). The year before that is 0000, which corresponds to 1 BC (1 BCE). The year before that is -0001, which corresponds to 2 BC. This pattern continues. This system had already been used by astronomers (see astronomical year numbering) and may clarify the dispute about when a new century begins (see 20th century).

Years may optionally be relaxed to fewer than four digits. However, unless ISO 8601 is assumed, this is potentially dangerous. For instance, 2006-01-09 could be written 060109. Years can also be expanded to greater than four digits, though this is practically domain specific—such as archeological and astronomical events.

ISO 8601 describes three date formats of general interest. These can be quickly distinguished by a one, two, or three digit day field.

ISO 8601 - Calendar date

Calendar dates are the form familiar to most people. As represented above, YYYY indicates a year with century, and a negative sign for years before 1 BCE. MM indicates the month of the year, 01 through 12. DD indicates the day of that month, from 01 through 31. For example, "April 5th, 1981" may be represented as 1981-04-05 in the extended format, or 19810405 in the basic format.

The standard allows for dates to be written with less precision. For example, you may write 1981-04 to mean "April, 1981". You may simply write 1981 to refer to that year.

The standard also allows for dates which include an implied element, such as an implied century. It is careful to emphasize the importance of clear communication between sender and receiver when implied elements are being used. It is a lack of such clarity that led to the year 2000 problem.

ISO 8601 - Week dates

Week dates are a form more popular in manufacturing. As represented above, YYYY indicates a year. The "W" is a literal roman character followed by ww which represents the week number from 01 through 52 or 53. The "D" represents the day of the week from 1 through 7, beginning with Monday and ending with Sunday. For example, 1981-04-05 is the 7th day of the 14th week of 1981, and would be written 1981-W14-7, or in its most compact form 81W147.

Week 01 is the week with the year's first Thursday in it. Or in other words, the first week with the majority (four or more) of its days in the new year. If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in week 01. If 1 January is on a Friday, Saturday or Sunday, it is in week 52 or 53 of the previous year. Weeks 01, 52 and 53 can thus include days from more than one year and, in most years, do so.

ISO 8601 - Ordinal dates

Ordinal dates are a simple form for times when the arbitrary nature of week and month definitions are more of an impediment than an aid—for instance, when comparing dates from different calendars. As represented above, YYYY indicates a year. DDD is the day of that year, from 01 through 366 in leap years. For example, "1981-04-05" is also 1981-095.

This format has particular use for simple hardware systems that have need of a date system, but where including full calendar calculation software may be a significant nuisance.

ISO 8601 - Further details about dates

The standard allows for expansion and truncation of the year, by agreement between sender and receiver. Truncation means the year may be written with two digits (optionally preceded by a hyphen) to represent a year in an implied century. Unfortunately, common practice is to use two digits for either a year in an implied modern century or in the first century (89 might be 89 or 1989). Expansion means that the year may be written with more than four digits, which addresses the year 10,000 problem, by allowing the standard to specify dates later than AD 10000 or earlier than 10001 BC. Note however that expansion and truncation introduce ambiguities if separators are not used. For instance 200406 could either mean the year 200406 or month 06 in year 2004. The ISO standard suggests that "provisions be made" to prevent such confusions.

For purposes of reference ISO 8601 assigns the number 1875 to the year in which the Convention du Mètre was signed in Paris.

Adapted from the Wikipedia article "Dates", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki