ISO 8601
Also known as: ISO 8601 date format, ISO date, RFC 3339
ISO 8601 is the international standard for writing dates and times as text — 2026-08-03T14:30:00Z — ordering every field from largest to smallest so the result is unambiguous worldwide and sorts correctly as a plain string.
Overview
The standard fixes the one thing local date formats cannot agree on: order. 03/08/2026 is 3 August to most of the world and 8 March in the United States, and no amount of context makes that safe. ISO 8601 writes year, then month, then day, then hour, minute, second — big-endian throughout — which has the useful side effect that sorting the strings alphabetically also sorts them chronologically. No parsing required to order a log file.
A full timestamp is a date, the letter T, a time, and an offset: 2026-08-03T14:30:00+02:00. 'Z' — Zulu — is shorthand for a +00:00 offset, meaning UTC. That trailing offset is the part people drop, and dropping it is what turns a timestamp into a guess: 2026-08-03T14:30:00 with no offset is a wall-clock reading with no instant attached, and different parsers will assume different things about it. Always include the offset.
An offset is also not a time zone. +02:00 tells you the arithmetic, not the location or the rules, so it cannot tell you what the offset will be six months later when daylight saving has shifted. If a future event needs to survive a DST change, store the IANA zone name — Europe/Berlin — alongside the timestamp.
In practice most APIs implement RFC 3339, a tightened internet profile of ISO 8601 that requires a 4-digit year, the separators, and an offset. The two overlap heavily but neither strictly contains the other: RFC 3339 permits '-00:00' for an unknown local offset, which ISO 8601 does not, while ISO 8601 allows plenty RFC 3339 omits — week dates (2026-W31-1), ordinal dates (2026-215), durations (P3Y6M4D), intervals, and a separator-free basic format (20260803T143000Z). Interoperate on the RFC 3339 subset and you will rarely be surprised.