ISO 8601 Date Formatter

Choose a date, a time, and a UTC offset and press Format date. You get the moment in ISO 8601 extended and basic forms, in UTC, as an ISO week and ordinal date, and in email, HTTP, Unix, and everyday formats.

Try an example

About this ISO 8601 formatter

ISO 8601 is the international standard for writing dates and times so that anyone, and any computer, reads them the same way: largest unit first, as in 2026-10-04T15:30:00+05:30. The extended form uses dashes and colons; the basic form drops them. RFC 3339, used across the web and in APIs, is a strict profile of ISO 8601, usually written in UTC with a Z. The standard also defines week dates such as 2026-W40-7 and ordinal dates such as 2026-277. This formatter takes a local date, time, and UTC offset and writes the same moment in all of these forms, plus the RFC 2822 format used in email headers, the HTTP date format, Unix timestamps, and common US and European styles.

How to format a date in ISO 8601

  1. Enter the date and time. Pick the date and the local time, with seconds if you need them, or press an example.
  2. Choose the UTC offset. Select the offset of the place the time belongs to, such as UTC+05:30 for India or UTC−05:00 for New York in winter.
  3. Press Format date. Check the color-coded breakdown and copy whichever format you need.

Frequently asked questions

What does the T in an ISO date mean?

It separates the date from the time, as in 2026-10-04T15:30:00. RFC 3339 also allows a space there, but T is the standard form.

What does a trailing Z mean?

Z stands for UTC (zero offset), sometimes read as “Zulu time”. 2026-10-04T10:00:00Z is the same moment as 2026-10-04T15:30:00+05:30.

What is the difference between ISO 8601 and RFC 3339?

RFC 3339 is a stricter subset of ISO 8601 for internet use: it requires a full date and time with an offset or Z. Every RFC 3339 timestamp is valid ISO 8601, but not the other way round.

How is the ISO week number worked out?

Weeks start on Monday, and week 1 is the week containing the year’s first Thursday. Dates in early January or late December can therefore belong to the neighboring year’s week.

Why does the UTC date sometimes differ from my date?

Converting to UTC can cross midnight. 23:00 at UTC+05:30 is 17:30 UTC on the same day, but 02:00 at UTC+05:30 is 20:30 UTC on the previous day.