ISO 8601

ISO 8601 is an international standard covering the worldwide exchange and communication (information interchange) of date- and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019. The purpose of standard ISO 8601 is to provide a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times. (The standard is also described by the terms: Information interchange; Interchange standard; Data elements and interchange formats; Representation of dates and times.)

Current date and time expressed according to ISO 8601 [refresh]
Date and time in UTC2021-09-27T17:08:45+00:00
Week with weekday2021-W39-1
Date without year--09-27[1]
Ordinal date2021-270

In general, ISO 8601 applies to these representations and formats: dates, in the Gregorian calendar (including the proleptic Gregorian calendar); times, based on the 24-hour timekeeping system, with optional UTC offset; time intervals; and combinations thereof.[2] The standard does not assign specific meaning to any element of the dates/times represented —such meaning being determined by the context of the specific use. And dates and times represented cannot use words that do not have a specified numerical meaning within the standard (thus excluding names of years in the Chinese calendar), or that do not use computer characters (excludes images or sounds).[2]

In representations that adhere to the ISO 8601 interchange standard, dates and times are arranged such that the greatest temporal term (typically a year) is placed at the left and each successively lesser term is placed to the right of the previous term. Representations must be written in a combination of Arabic numerals and the specific computer characters (such as "-", ":", "T", "W", "Z") that are assigned specific meanings within the standard; that is, such commonplace descriptors of dates (or parts of dates) as "January", "Thursday", or "New Year's Day" are not allowed in interchange representations within the standard.