ISO 8601 Validator + Builder
Validate any ISO 8601 timestamp and see it expanded into every equivalent form, or build one interactively from a date, time, and timezone.
Frequently asked questions
What is ISO 8601?
ISO 8601 is the international standard for representing dates and times as strings. It defines the YYYY-MM-DD calendar form, the HH:MM:SS time form, the T separator that joins them, and the +HH:MM or Z suffix that anchors the timestamp to a UTC offset. Most modern APIs and databases expect ISO 8601 strings because they sort correctly as plain text and are unambiguous across locales.
What is the difference between basic and extended format?
Extended format uses separators — hyphens between date parts, colons between time parts, like 2026-06-02T18:42:23. Basic format strips the separators, like 20260602T184223. Both are valid ISO 8601, but extended is by far the more common form in APIs and human-facing software. Basic format is mostly used in older protocols and some compact serialization formats.
Is RFC 3339 the same as ISO 8601?
RFC 3339 is a profile of ISO 8601 with stricter rules: it always requires the T separator (though it also permits a space as an alternative), always requires a timezone offset, and disallows ISO 8601 features like week dates and ordinal dates. If your spec says RFC 3339, an ISO 8601 string will validate only if it sticks to the calendar-date + extended-time + offset combination.
When should I use the "Z" suffix vs an offset like +05:30?
Use Z when the time is in UTC and you want to make that explicit — it is shorter and unambiguous. Use a numeric offset when the time was captured in a specific local zone and the offset is meaningful for the consumer (audit logs, calendar events, anything tied to a place). Both are valid; the parsed DateTime is identical, only the representation differs.
What are week dates and ordinal dates?
Week dates encode the year, ISO week number, and weekday: 2026-W23-2 means Tuesday of week 23, 2026. Ordinal dates encode the year and day-of-year: 2026-153 means the 153rd day of 2026 (June 2nd). Both are valid ISO 8601 calendar variants but are rarely used in practice — most consumers expect the standard YYYY-MM-DD form.
Does ISO 8601 allow milliseconds and microseconds?
Yes. Fractional seconds are written after a period: 18:42:23.456 (milliseconds), 18:42:23.456789 (microseconds), 18:42:23.456789123 (nanoseconds). The standard places no limit on precision, but JavaScript Date only retains millisecond precision, so anything beyond 3 digits is truncated by most browser APIs.