What this ISO 8601 converter does
ISO 8601 is the international standard for writing dates and times so that a computer in Tokyo and a computer in Toronto read them the same way. This tool works in both directions: give it a date, time and timezone and it builds the correctly formatted string; paste in a string you got from an API or a log file and it decodes it into a readable date, a Unix timestamp, and two lesser-known ISO 8601 notations — the week date and the ordinal date.
How to use it
Pick a direction, then fill in the fields — everything else updates live.
- Choose "Convert a date" or "Decode a string."Building starts from a date and time; decoding starts from an existing ISO 8601 string.
- Fill in the details.For building: a date, a time (with seconds), optional milliseconds, and UTC or your browser's local timezone. For decoding: paste the string as given.
- Read the result.The ISO 8601 string, Unix timestamp, and day of week appear instantly, with a warning if the input isn't valid.
- Check the alternate notations.The same instant is also shown as a week date and an ordinal date, both defined by the ISO 8601 standard.
How ISO 8601 date-times are structured
The full extended format is YYYY-MM-DDTHH:MM:SS±HH:MM — a calendar date, a literal T separating it from the time, a 24-hour time, and a timezone offset (or Z for UTC). Any trailing component can be dropped: a date on its own, like 2026-08-28, is a complete and valid ISO 8601 value.
2026-08-28, Time = 14:30:00, Offset = +05:30ISO 8601 = 2026-08-28T14:30:00+05:30
Converted to UTC = 2026-08-28T09:00:00Z (subtract the +5:30 offset)
Unix timestamp = 1787900400
Week dates and ordinal dates
ISO 8601 also defines two alternate ways to write the same calendar day. A week date (2026-W35-5) counts Monday-starting weeks, where week 1 is the week containing the year's first Thursday — this is why the "ISO week year" occasionally differs from the calendar year in late December or early January. An ordinal date (2026-240) simply counts the day of the year, from 001 to 365 or 366.
Timestamps without a timezone
A date-time with no offset is still valid ISO 8601, but it doesn't pin down an exact instant — the standard leaves its meaning to the surrounding context. This converter assumes UTC in that case and says so, rather than silently guessing at your local timezone. For the epoch-seconds equivalent of any instant, the Unix timestamp converter and epoch converter cover that conversion on their own.
Quick examples
Common inputs and the ISO 8601 string each one produces.
| Input | ISO 8601 (UTC) |
|---|---|
| Aug 28, 2026, midnight | 2026-08-28T00:00:00Z |
| Aug 28, 2026, 2:30 PM | 2026-08-28T14:30:00Z |
| Aug 28, 2026 (date only) | 2026-08-28 |
| Aug 28, 2026, 2:30:45.500 PM | 2026-08-28T14:30:45.500Z |
| Jan 1, 2026, midnight | 2026-01-01T00:00:00Z |
| Dec 31, 2026, 11:59:59 PM | 2026-12-31T23:59:59Z |
Who uses it, and for what
- API developers formatting or debugging timestamps in request and response payloads.
- Anyone reading log files that timestamp entries in ISO 8601, wanting a quick readable date and Unix timestamp.
- Database and analytics work where dates need to move between a calendar picker and a stored ISO string.
- Documentation & QA — checking that a generated timestamp is actually valid before it ships.
Scheduling a recurring job instead of formatting a single instant? The cron expression generator covers that. For a broader look at UTC offsets around the world, see the UTC converter.
Frequently asked questions
What is ISO 8601?
ISO 8601 is the international standard for representing dates and times unambiguously, such as 2026-08-28T14:30:00Z. It defines the field order (year, month, day, hour, minute, second), the separators, and how to express a timezone offset.
What's the difference between "Z" and "+00:00"?
Nothing functionally — both mean an offset of zero from UTC. "Z" (short for "Zulu time") is simply a shorter, equivalent way of writing "+00:00".
Does ISO 8601 require a timezone offset?
No. ISO 8601 allows a date-time without an offset, but its meaning is then implementation-defined rather than a specific instant. Including an offset (or "Z" for UTC) is strongly recommended for unambiguous exchange, and this tool assumes UTC when one is missing.
What are ISO week dates and ordinal dates?
Alongside the familiar calendar date, ISO 8601 defines two other notations for the same day: the week date (YYYY-Www-D), where weeks start on Monday and week 1 contains the year's first Thursday; and the ordinal date (YYYY-DDD), which counts the day of the year from 001 to 365 or 366.
Why does JavaScript parse some ISO 8601 strings differently?
A date-only string like "2026-08-28" is parsed by JavaScript's Date() as UTC midnight, but a date-time string without an offset, like "2026-08-28T14:30:00", is parsed as local time. This inconsistency is a known source of bugs — always include an explicit offset or "Z" when precision matters.
Is millisecond precision required in ISO 8601?
No. ISO 8601 allows any number of decimal digits on the seconds field, or none at all. Milliseconds (three digits) are simply the precision most programming languages default to.
Is this converter free and private?
Yes — it's free with no sign-up, and everything runs in your browser. Nothing you enter is uploaded or stored on a server.
Accuracy & sources
Building an expression assembles it directly from the date, time and timezone you set — no hidden translation step. Decoding validates the string against the ISO 8601 extended date-time pattern, checks every field is in range, and confirms it's a real calendar date (catching things like February 30th) before converting it. Every worked example on this page has been independently verified by hand.
The date-time format, the "Z" designator, and the week-date and ordinal-date notations all follow ISO 8601, the international standard for representing dates and times. Written and reviewed by Fletcher Langley, and re-checked whenever the tool changes — last reviewed August 2026.
About the author
Operations and workforce-systems specialist with 14+ years inside shift-based industries and payroll platforms, and a BS in Industrial Engineering. Fletcher writes and reviews the calculators, converters, and guides here — from overnight-shift math to decimal payroll hours — drawing on years spent translating between clock formats and date-time standards for real systems.
Written & reviewed by Fletcher Langley · Last updated: August 2026