ISO 8601 & RFC 3339

Validate any date string against ISO 8601 / RFC 3339, or build one from parts.

Date string to validate

Try an example

ISO 8601 vs RFC 3339

ISO 8601 is the international standard for date/time strings. RFC 3339 is a profile of ISO 8601 used in internet protocols (HTTP, email headers, JSON APIs) — it requires a timezone offset and uses only the T separator. Every valid RFC 3339 string is valid ISO 8601, but not vice versa.

Common use cases

REST APIs typically require RFC 3339 (2026-05-27T10:30:00Z). Databases often store ISO 8601 without offset. Use the builder to generate the exact format your system needs.