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ISO 8601 Converter

Convert a date into every ISO 8601 form, including week and ordinal dates.

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Quick Tool Overview

The ISO 8601 Converter gives every representation of one moment, including the week date — where a week belongs to the year holding its Thursday, so 1 January 2021 is in week 53 of 2020 and any report grouping by week and year separately is wrong.

What it does
Converts a date into every ISO 8601 form
You give it
A date, an epoch, or any recognised string
You get back
Extended, basic, week, ordinal and epoch
Best for
APIs, logs and anything crossing a timezone
Developer Tools Free, unlimited use No sign-up Runs in your browser

About the ISO 8601 Converter

Enter a date in almost any form — ISO, an epoch timestamp, an HTTP date — and get every ISO 8601 representation of that same moment, with what each one is for.

The week date is the part worth having. ISO week numbering does not line up with the calendar year: a week belongs to whichever year holds its Thursday, so 1 January 2021 falls in week 53 of 2020, and 30 December 2019 is already in week 1 of 2020. Any report that groups by week and by year independently will be wrong at those boundaries. The ordinal date — the day number within the year — is here too, along with the Unix epoch in seconds and milliseconds.

Key features of the ISO 8601 Converter

The week date, correctly

Computed by the Thursday rule, which is what makes the year boundary behave the way the specification requires rather than the way it looks.

Ordinal dates

The day number within the year, which some systems and most scientific data use in place of a month.

Says how it read you

ISO, epoch seconds, epoch milliseconds or a recognised string — so a misread input is visible immediately.

Epoch both ways

Seconds and milliseconds, generated and accepted, with the digit count used to tell them apart.

Local and UTC

The same instant with your offset and as Zulu time, side by side.

How to use the ISO 8601 Converter

  1. 1

    Enter a date

    ISO 8601, an epoch timestamp, or any date string your browser recognises.

  2. 2

    See how it was read

    The page says which format it recognised, so a misread is obvious.

  3. 3

    Read every form

    Extended, basic, week date, ordinal date, date only, time only and the epoch.

  4. 4

    Copy the one you need

    Click any value to take it.

Frequently asked questions

Zulu time — UTC, with no offset. A timestamp ending in Z is unambiguous anywhere in the world, which is why it is the right thing to store.

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