ISO 8601 (Date and Time Format)
ISO 8601 is the international standard for representing dates and times unambiguously — YYYY-MM-DD, the T separator, time-zone offsets, durations like P1Y2M, week dates, and intervals. RFC 3339 is the strict internet subset used in APIs.
The formats you will meet
Basic date: 2026-10-22 (year-month-day — the ordering that sorts correctly as text). Date-time: 2026-10-22T14:30:00+02:00 or with Z for UTC. Durations use P: P1Y2M3DT4H (one year, two months, three days, four hours). Week dates (2026-W43-4) and intervals (start/end or start/duration) round out the standard.
RFC 3339 narrows the standard for internet protocols: it requires the full year-month-day, the T separator, and an explicit offset — no durations, no week dates, no reduced precision. When an API spec says ISO 8601, it almost always means RFC 3339.
Why format discipline matters
Ambiguous dates corrupt data silently: 03/04/2026 means April 3 in the US and March 4 in Europe. ISO 8601 removes the ambiguity, sorts lexicographically, and survives round-trips through CSVs, databases, and message formats — which is why payment files, FHIR resources, and log pipelines all standardize on it.
Related Standards
- •ISO 8601-1:2019 — date and time representations
- •RFC 3339 — the internet-profile subset
Free Tools & Resources
Further Reading
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