ISO/TC 154

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Introduction to the new ISO 8601-1 and ISO 8601-2

ISO 8601-1:2019 and ISO 8601-2:2019 have been published simultaneously in March 26 2019, as successors of the widely successful ISO 8601:2004.

One of the most applied standards across the world, ISO 8601 is the International Standard for representing date and time for interchange.

In this post we briefly describe the history of the ISO 8601 family of standards and explain how organizations and businesses can take advantage of the new features offered in the new editions.

An accurate interchangeable date and time mechanism is crucial to the function of today’s world. Trade, travel, transport, transactions, and legal all rely on the accuracy and comparability of date and time.

ISO 8601 is the International Standard for date and time representations

The ISO 8601 family of standards belong under the scoped title of “Date and time — Representations for information interchange”.

Do you recognize these date and time representations?

  • 2019-03-26

  • 2019-03-26T01:20:33

  • 2019-03-26T03:20:33Z

  • 2019-03-26T11:20:33+0800

Probably. These are representations defined by ISO 8601. ISO 8601 and its predecessors have been the golden standard for representing date and time since the 1970s.

ISO 8601 is managed by ISO/TC 154 and developed under its WG 5

We encourage developers and related stakeholders to get familiar with the latest developments and features provided by these standards—we believe that these standards will be widely adopted.

— Ms Hongru (Judy) Zhu; Chair of ISO/TC 154

ISO/TC 154 was created in 1972 to covers the scope of “Processes, data elements and documents in commerce, industry and administration”.

One of the most important matters in the scope is to deal with interoperable date and time encoding.

The Chairperson of ISO/TC 154 is Ms Hongru (Judy) Zhu, with its secretariat run by SAC (Mr Jianfang Zhang, Committee Manager).

Its Working Group 5 (ISO/TC 154/WG 5) named “Date and time” is responsible for ISO’s date and time efforts.

A short history of ISO 8601

Here’s a short history of ISO 8601.


  • ISO 2014:1976 (all-numeric dates)

  • ISO 2015:1976 (week numbering)

  • ISO 2711:1973 (ordinal date numbering)

  • ISO 3307:1975 (representations of time of the day)

  • ISO 4031:1978 (time differentials)

These standards were all superseded by the first ISO 8601, ISO 8601:1988.

And subsequently ISO 8601 has been updated multiple times:

  • ISO 8601:1988

  • ISO 8601:1988/Cor 1:1991

  • ISO 8601:2000

  • ISO 8601:2004

Motivation for revising ISO 8601:2004

ISO 8601:2004 was in need of revision and improvement.

Representations in the standard needed more formal definitions. The document contained numerous instances that were defined by lax adherence, with conflated concepts of expression and representation. For example, in the representation format YYYY-MM-DDTHH:MM:SS, the usage of T is different from Y. T is to be directly expressed, but Y, M etc. are used as placeholders (variables). While this is typically understood by users, it presents a challenge for encoding these representation formats in computer use.

Some rules used in ISO 8601:2004 led to unnecessary ambiguity. For example, the “nominal duration of month” was set to 30 days, but real-life usage differs.

The document also did not provide provision for representing individual date-time components. For example, a “time shift” cannot be expressed on its own (e.g. +0800, UTC+8).

Last but not least, users have been demanding that ISO 8601 “does more” — there are many instances where expressing further meaning is necessary. For example, the need for expressing years beyond 4-digits, negative years, specifying moving days, recurring events are all necessary.

ISO/TC 154 undertook a modernization effort of ISO 8601 in 2017

All earlier versions of ISO 8601 are single-part standards.

ISO 8601 is now a family of standards:

  • ISO 8601-1:2019 is the direct successor to ISO 8601:2004

  • ISO 8601-2:2019 provides extensions on top of ISO 8601-1:2019

Key changes in 2019 ISO 8601-1 and ISO 8601-2

ISO 8601-1 ISO 8601-2
  • Heavily rewritten (>80%)

  • Component-based, 40+ newly defined terms, normatively referenced by BIPM (creator of UTC)

  • Titled “Date and time — Representations for information interchange — Part 1: Basic rules”

  • FDIS finalized in 2018-12

  • Completely new!

  • Enables new usage of date and time representations

  • Titled “Date and time — Representations for information interchange — Part 2: Extensions”

  • FDIS finalized in 2019-01

  • ISO 8601-1, ISO 8601-2 both published 2019-03-26

ISO 8601-1: Terminology and conceptual clarification

date and time representation

format syntax representing date and time in general, shown within brackets

date and time expression

instantiation of a representation that represents a date and time, shown within single quotes

time scale unit

a measurement unit of the time scale on the time-axis

time scale component

a representation of the time scale unit in a date and time representation

duration of a time scale unit

e.g. “day”

the “reference” that points to a time interval

e.g. “calendar day”

digit replacements

such as “YYYY”, where each “Y” represents a digit in the year component)

designator symbols

such as “["Y"]” which is the text character “Y”)

ISO 8601-1: Important new terms standardized for the first time internationally


time ( on the calendar ( time scale (

time shift

constant duration ( difference between times ( of two time scales (


time scale ( that uses the time scale unit ( of calendar day ( as its basic unit

week calendar

calendar ( in general use that defines a calendar year ( that approximates the tropical year using a calendar week ( as its basic unit


time scale ( suited for intra-day time measurements

24-hour clock

clock ( that subdivides a calendar day ( into 24 clock hours (

ISO 8601-1: Everything is now defined from components

  • Every time scale component is now individually defined.

  • Time scale components are now given “variables” and re-used heavily, unlike previously where each representation had to list out full specifications.

  • Time shift” is now a date and time representation that can be used individually.

  • No more “nominal duration”, just “exact duration”.

  • No more “time interval specified by a duration and context information”.

  • Addition of “decade and century representation” (from EDTF).


Prepared by:

  • Ray Denenberg (Library of Congress; Convenor ISO/TC 154/WG 5 2018-2019)

  • Ronald Tse (Ribose, CalConnect; Convenor ISO/TC 154/WG 5 2019-)

International Organization for Standardization
ISO Central Secretariat
Chemin de Blandonnet 8 Case Postale 401
CH – 1214 Vernier, Geneva

26 August 2019