Understanding ISO 8601

A common notation for exchanging dates and times, provided their context is explicit.

Break down a date

2024-04-01T14:00:00+02:00 means April 1, 2024 at 14:00, with a two-hour offset from UTC. T separates the date from the time; Z can replace +00:00 to denote UTC.

A string without an offset does not always identify an instant

2024-04-01 is a calendar date. 2024-04-01T14:00:00 is a date and time without an offset. Without an additional convention, these strings are not enough to identify a universal instant.

Fractions of a second

2024-04-01T12:00:00.123Z adds 123 milliseconds. Precision limits depend on the library: JavaScript Date preserves milliseconds, while Python datetime can preserve microseconds.

ISO 8601 and APIs

ISO 8601 includes several representations, including calendar dates, week dates and durations. APIs often use a subset close to RFC 3339. Check your API contract rather than assuming every form is supported.

Test an ISO date

References

RFC 3339 — dates and times on the Internet

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English version published September 26, 2026.