ISO 8601 date format with timezone offset

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Short answer. Write instants as YYYY-MM-DDTHH:MM:SS.sss+HH:MM or with Z for UTC (2026-09-10T08:05:31.190Z). Write calendar dates as YYYY-MM-DD with no time. Never write an offset-less time when the instant matters.

Forms

Value Meaning
2026-09-10 A calendar date, no instant
2026-09-10T08:05:31Z An instant in UTC
2026-09-10T10:05:31+02:00 The same instant, written in a +02:00 zone
2026-09-10T08:05:31 Ambiguous local time; avoid for data exchange
2026-W37-4 ISO week date (Thursday of week 37)
P3DT4H A duration of 3 days 4 hours

Details

  • RFC 3339 is the internet profile of ISO 8601; it is what JSON APIs mean by "ISO date".
  • An offset is not a zone: +02:00 does not tell you whether DST applies next month. Store the IANA zone name (Europe/Berlin) separately when you need local rules.
  • Sorting ISO strings lexically sorts by time only when they share an offset; normalize to UTC first.

Sources