ISO 8601 / RFC 3339 Date Parser & Formatter
Parse any ISO 8601 form — date-time with offset, week or ordinal date, duration, Unix epoch — and get UTC ISO, RFC 3339, RFC 2822, epoch, ISO week, wall clock.
The exact instant a timestamp means, written every way a spec or a log might want it, with the ISO week and day of year, the offset that was given, and a warning when none was.
Example: 2026-09-19T14:30:05.123+07:00 is 2026-09-19T07:30:05.123Z, epoch 1789803005, ISO week 2026-W38-6, day 262 of the year, a Saturday — and 14:30:05 on the wall clock in Asia/Jakarta.
One instant,
many spellings.
What the parser accepts, how the instant is fixed, and how the derived forms are computed.
Accepted forms
Calendar dates (2026-09-19, 20260919, 2026-09), date-times with T or a space, seconds and fractions optional, and an offset of Z or ±hh:mm; ISO week dates (2026-W38-6); ordinal dates (2026-262); durations (P1Y2M3DT4H5M6S, PT90M, P2W); and Unix epochs in seconds or milliseconds (told apart by magnitude). Impossible dates such as 2026-02-30 are refused rather than rolled over.
Fixing the instant
A date-time with an offset is converted to UTC by subtracting the offset. A date-time with no offset is ambiguous; the page takes it as UTC and says so, because guessing your zone silently is how off-by-hours bugs happen. A bare date is midnight UTC. From the UTC instant come the ISO extended and basic forms, RFC 3339 (with +00:00), the RFC 2822 / HTTP-date form, the epoch, and the wall clock in the IANA zone you choose, rendered by the browser's Intl data with that zone's offset at that moment (so daylight-saving is respected).
Weeks, ordinals, durations
ISO weeks start on Monday and week 1 is the week containing 4 January, so 1 January can fall in week 52 or 53 of the previous week-year — the page shows the week-year separately. The ordinal day counts from 1 January. Durations are reported by component; a total in seconds is given only when no years or months are present, since those have no fixed length. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- ISO 8601-1:2019 — date and time representations (calendar, week and ordinal dates, durations); cited as the reference, not reproduced
- RFC 3339 — Date and Time on the Internet: Timestamps
- RFC 5322 §3.3 — date-time format (RFC 2822 style)
Last reviewed 19 September 2026. How results are checked: How we verify.