ISO Date Converter

What this tool does

Paste a date in almost any shape — an ISO string, an email header date, a bare epoch number, something a database printed — and this page tells you which instant it names and rewrites it in every format you are likely to need. It also works the other way: set the components, choose an offset, and get a string a strict RFC 3339 parser will accept.

It never silently guesses about the offset. If your input has no Z and no +hh:mm, the page says so and shows which zone it assumed, because that one decision is what moves a date by up to 26 hours.

Common uses

• Turning a database timestamptz dump into a value an API will accept.
• Checking whether a string is genuinely RFC 3339 before sending it to a strict parser such as Go's time.RFC3339 or a JSON Schema format: date-time.
• Working out which ISO week a date falls in for a reporting query.

A short example

Input:

Tue, 14 Nov 2023 22:13:20 +0000

Output:

ISO 8601 (UTC)   2023-11-14T22:13:20.000Z
ISO basic        20231114T221320Z
ISO week date    2023-W46-2
ISO ordinal      2023-318
Unix seconds     1700000000

Z, +00:00, and the difference between ISO 8601 and RFC 3339

Z is short for "Zulu", the military name for the zero meridian. In 2026-09-24T14:30:05Z it means the offset from UTC is zero. Writing +00:00 names the same instant, so the two are interchangeable for arithmetic — but RFC 3339 §4.3 draws a distinction of intent: -00:00 is reserved for "this is UTC, and the local offset is unknown", whereas +00:00 asserts the local offset genuinely is zero. A real offset such as +02:00 carries more still: it pins the instant and records what the clock said locally, which is why it beats 12:30:05Z in an audit trail.

RFC 3339 is a deliberately narrow profile of ISO 8601 for the internet. It requires a full date and time, requires an offset, and forbids most of ISO 8601's flexibility — no week dates, no ordinal dates, no omitted fields, no 20260924T143005 basic format. Practically everything RFC 3339 accepts is valid ISO 8601; a great deal of valid ISO 8601 is not valid RFC 3339. RFC 3339 does permit a space in place of T by explicit agreement (§5.6); ISO 8601 does not.

Worth knowing

Offsets are not timezones. +02:00 says the clock was two hours ahead of UTC at that moment; it does not say whether that was Berlin in summer or Cairo in winter, and it cannot say what the offset will be next March. For anything scheduled in the future, store the IANA zone name — Europe/Berlin — alongside the local time, because governments change DST rules with weeks of notice. An offset is right for an event that has already happened; a zone name is right for one that has not.