ISO 8601 Date Converter is a free tool that checks whether a date is valid ISO 8601 and converts it to every common ISO form: with a time zone offset, in UTC, as a week date, as an ordinal date and in basic format. It also reads ISO 8601 durations such as P1DT2H and explains them in plain English. It runs in your browser.
How to use the converter
- Paste an ISO 8601 date or time, a duration, a Unix timestamp, or click Current time.
- Choose the time zone for the “With offset” result.
- Copy the format you need.
ISO 8601 formats at a glance
| Example | Meaning |
|---|---|
2026-09-27 | A calendar date |
2026-09-27T14:30:00Z | Date and time in UTC |
2026-09-27T18:30:00+04:00 | The same moment, written for a zone 4 hours ahead of UTC |
2026-W39-7 | Week date: Sunday (day 7) of week 39 |
2026-270 | Ordinal date: the 270th day of 2026 |
20260927T143000Z | Basic format, without separators |
PT1H30M | A duration of 1 hour 30 minutes |
Durations
ISO 8601 durations are used by APIs, video platforms and calendar files. They start with P(period), list years, months, weeks and days, then T and hours, minutes and seconds. Watch out forM: before the T it means months, after it minutes, so P1M is one month andPT1M is one minute. The converter gives totals in seconds, hours and days, counting a month as 30 days where months are involved.
Local time pitfalls
A date-time without Z or an offset, such as 2026-09-27T14:30, is local time, and each computer reads it in its own time zone. That is a common source of off-by-hours bugs, so the converter tells you when a value has no zone. In JavaScript, new Date().toISOString() always produces UTC with milliseconds, as in the “UTC with milliseconds” result.
To work with Unix time, use the Unix Timestamp Converter. To count the days between two dates, use the Date Difference Calculator.
Frequently asked questions
What is ISO 8601?
The international standard for writing dates and times: year first, then month and day, as in 2026-09-27T14:30:00Z. Because it reads the same everywhere and sorts correctly as text, APIs, databases and log files use it.
What do the Z and +04:00 mean?
They give the time zone offset from UTC. Z means UTC itself (“Zulu time”); +04:00 means four hours ahead of UTC. A time without either is “local time”, which is ambiguous between systems, so APIs should always include one.
What is an ISO 8601 duration?
A length of time written as P, then years (Y), months (M), weeks (W) and days (D), then T and hours (H), minutes (M) and seconds (S). P1Y2M10D is 1 year, 2 months and 10 days; PT1H30M is 1 hour 30 minutes.
What is an ISO week number?
Weeks start on Monday, and week 1 is the week containing the year’s first Thursday. So 1 January can belong to the previous year’s week 52 or 53: 1 January 2027 is 2026-W53-5.
Is ISO 8601 the same as RFC 3339?
RFC 3339 is a stricter profile of ISO 8601 for the internet. It requires a full date, time and offset, such as 2026-09-27T14:30:00+04:00. The “With offset” and “UTC” results here are valid in both.
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