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Note that this is only a brief description of the standard, not the standard itself (for which the copyright is held by the ISO).
The standard defines formats for numerical representation of dates, times and date/time combinations. Local time and Coordinated Universal Time (UTC) are supported. Dates are for the Gregorian calendar (introduced in 1582), and can be given in year-month-day, year-week-day or year-day formats. Times are given in 24hr format. All date and time formats are represented with the largest units given first, i.e., from left to right the ranking is year, month, week, day, hour, minute, second. Any particular date/time format is a subset of these possible values, and the standard lists various permissible subsets. The hyphen character is sometimes used to represent missing components, thus avoiding ambiguity. The standard allows these hyphens to be omitted by mutual agreement in a data interchange, if ambiguity can be avoided. Formats are also given for periods of time, either by listing the start and end dates or as a given length of time.
Unless otherwise stated, all values are fixed width, with leading zeros used when necessary to pad out a value.
Combined Date/Time Formats
The symbol "T" is used to separate the date and time parts of the combined representation. This may be omitted by mutual consent of those interchanging data, if ambiguity can be avoided. The complete representation is as follows:
Periods of Time
Where two time values are required, they are separated with a solidus "/", although the standard states that a double hyphen is used in "certain application areas". The dates are given in calendar form, but ordinal or week/day may alternatively be used, and basic representation can be replaced by the appropriate extended representation. Likewise, valid reduced precision, truncated or decimal formats may be used. Periods can also be prefixed with a P and identify Years, months, weeks and days. The value starts with "P", and is followed by a list of periods, each appended by a single letter designator: "Y" for years, "M" for months, "D" for days, "W" for weeks, "H" for hours, "M" for minutes, and "S" for seconds. Time components must be preceded by the "T" character.
For example, a calendar year may be represented by:
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