What exactly does ISO 8601 require?

It's an international standard for representing dates and times unambiguously: YYYY-MM-DD for dates, HH:MM:SS for times, a T separating them, and either Z for UTC or a ±HH:MM offset for other timezones — like 2026-10-04T14:30:00Z. The standard also defines less common forms like week dates (2026-W40-2) and ordinal dates (2026-277).

Why does my timestamp look different in UTC vs. local time?

The same instant in time is written differently depending on which timezone's offset you apply — 2026-10-04T14:30:00Z and 2026-10-04T10:30:00-04:00 represent the exact same moment, just expressed relative to UTC versus an Eastern Time offset. Toggling between UTC and local here lets you see both representations of the same instant.

What's an ISO week date, and why does the week number sometimes feel off?

ISO weeks always start on Monday, and week 1 is defined as the week containing the year's first Thursday — which means the last few days of December or the first few days of January can belong to a week numbered for the adjacent year. This often differs from the week number shown in a typical calendar app.

Why would I need the ordinal date format?

An ordinal date (YYYY-DDD) expresses a date as the year plus its day-of-year number, which is useful in scientific logging, manufacturing date codes, and any system where sorting or computing day differences by a single number is simpler than parsing month/day pairs.

ISO 8601 DATE CONVERTER

Build or parse an ISO 8601 date/time, with week date, ordinal date, and Unix timestamp forms included.

Date + time	2026-10-06T17:49:00Z
With milliseconds	2026-10-06T17:49:00.000Z
Date only	2026-10-06
Week date	2026-W41-2
Ordinal date	2026-279
Unix timestamp	1791308940
RFC 2822	Tue, 06 Oct 2026 17:49:00 GMT

How to convert a date to ISO 8601

1
Pick a date/time with the picker, or switch to pasting an existing date string in almost any common format.
2
Choose UTC or your local timezone offset.
3
Read the result as a full ISO 8601 date-time, date-only, ISO week date, ordinal date, Unix timestamp, and RFC 2822 string.

Questions

What exactly does ISO 8601 require?: It's an international standard for representing dates and times unambiguously: YYYY-MM-DD for dates, HH:MM:SS for times, a T separating them, and either Z for UTC or a ±HH:MM offset for other timezones — like 2026-10-04T14:30:00Z. The standard also defines less common forms like week dates (2026-W40-2) and ordinal dates (2026-277).
Why does my timestamp look different in UTC vs. local time?: The same instant in time is written differently depending on which timezone's offset you apply — 2026-10-04T14:30:00Z and 2026-10-04T10:30:00-04:00 represent the exact same moment, just expressed relative to UTC versus an Eastern Time offset. Toggling between UTC and local here lets you see both representations of the same instant.
What's an ISO week date, and why does the week number sometimes feel off?: ISO weeks always start on Monday, and week 1 is defined as the week containing the year's first Thursday — which means the last few days of December or the first few days of January can belong to a week numbered for the adjacent year. This often differs from the week number shown in a typical calendar app.
Why would I need the ordinal date format?: An ordinal date (YYYY-DDD) expresses a date as the year plus its day-of-year number, which is useful in scientific logging, manufacturing date codes, and any system where sorting or computing day differences by a single number is simpler than parsing month/day pairs.