ISO 8601 dates explained

What the standard covers

ISO 8601-1:2019 has the title Date and time, Representations for information interchange, Part 1, Basic rules ¹. ISO says it specifies representations of dates of the Gregorian calendar and times based on the 24-hour clock, and that it applies to times and time shifts based on Coordinated Universal Time ¹.

The standard leaves some things out. ISO lists date elements from non-Gregorian calendars, times not based on the 24-hour clock and character encoding as outside its scope ¹. The first edition of Part 1 was published in February 2019, was reviewed and confirmed in 2024 and has a 2022 amendment ¹.

The basic patterns

A W3C note from 1997 sets out a simple profile of ISO 8601 ³. It shows six levels of precision, from a year alone to a time with fractions of a second ³.

Time zones and offsets

The W3C note defines the time zone designator as Z or +hh:mm or -hh:mm ³. Z means UTC. An offset says how far local time is ahead of or behind UTC ³. Its example is 1994-11-05T08:15:30-05:00, which is 8:15:30 a.m. on 5 November 1994, US Eastern Standard Time, and the same instant as 1994-11-05T13:15:30Z ³.

RFC 3339 adds a rule for an unknown local offset. Where the time in UTC is known but the local offset is not, it can be written with an offset of -00:00 ². The RFC says Z or +00:00 imply that UTC is the preferred reference point for the time ².

RFC 3339 for internet records

RFC 3339, Date and Time on the Internet, Timestamps, was published in July 2002 by G. Klyne and C. Newman ². Its abstract calls it a profile of the ISO 8601 standard for dates and times using the Gregorian calendar ². It is a Proposed Standard and has been updated by RFC 9557 ².

The RFC joins a full date and a full time with the letter T, and its note allows a space in place of the T for readability ². It says that as the parts run from least precise to most precise, date and time strings may be sorted as strings, provided zones and fractional digits match ².

Why it matters for a dated record

Written dates such as 03/04/2026 can mean 3 April or 4 March depending on the reader. A YYYY-MM-DD string removes that doubt and sorts correctly ^2,3. For a timeline, store the date in ISO form, keep the reduced precision when the source gives only a year or a month, such as 2024-03, and record the offset when the source gives a time ³.