ISO 8601, the date format that sorts itself

By Niraj Kumar · Published October 28, 2026

Ask three people to write the fourth of March and you may get 04/03, 03/04 and 4 Mar. ISO 8601 exists to end that: it is the international standard for writing dates and times as text, and it orders every part from the largest unit to the smallest. The fourth of March 2026 is 2026-03-04, and there is no other reading of it. The date format converter turns any common notation into this one and back.

Dates

The full calendar date is YYYY-MM-DD, with every field zero-padded. The hyphens can be dropped in what the standard calls the basic format, 20260304, which is used in file names and in places where punctuation is awkward. Reduced precision is allowed by truncating from the right: 2026-03 is a month, 2026 a year. You cannot drop the month and keep the day, because a date has to be unambiguous when read left to right.

Because the largest unit comes first, ISO dates sort correctly as plain text. A directory of files named by ISO date lists in chronological order with no parsing at all. That is the property that makes the format worth adopting even where nobody else will ever read it.

Times

Times are hh:mm:ss on the 24-hour clock, with optional fractional seconds after a decimal point: 14:30:00, 14:30:00.250. A date and a time are joined with a capital T, so a full timestamp is 2026-03-04T14:30:00. The standard permits a space in place of the T by agreement between the parties exchanging data, and many systems accept one, but the T is what you should write. Midnight at the end of a day may be written as 24:00:00, which is the same instant as 00:00:00 on the next date; most software does not accept it, and it is better avoided.

Offsets and the letter Z

A time on its own is a local time with no indication of where. To fix it to an instant, append an offset from UTC: 2026-03-04T14:30:00+01:00 means half past two in a zone one hour ahead of UTC, which is 13:30 UTC. The offset is written with a sign, hours and minutes, and the minutes matter because of zones such as India’s +05:30 and Nepal’s +05:45. A time in UTC itself takes the suffix Z, 2026-03-04T13:30:00Z, from the military designation for the zero meridian, pronounced “Zulu”. The UTC clock shows the current time in this form.

Note what an offset is not: it is not a time zone. +01:00 might be Paris in winter or London in summer, and it says nothing about which rules will apply next month. An offset pins down one instant exactly; it does not identify the zone, and the guide to storing dates in software explains why that distinction decides how future events should be recorded.

Week dates and ordinal dates

The same standard defines the week numbering that payroll and logistics systems use. 2026-W10-3 is the third day (Wednesday) of week 10 of 2026. Weeks start on Monday and week 1 is the one containing the first Thursday of the year, which has consequences at the year boundary that the week number guide works through. Ordinal dates count the day of the year directly: 2026-063 is the 63rd day, which is 4 March. They are rare outside astronomy and some government forms.

Durations and intervals

A duration begins with P and lists units with their letters: P1Y2M10DT2H30M is one year, two months, ten days, two hours and thirty minutes. The T separates date units from time units, so that M can mean months before it and minutes after it. PT15M is fifteen minutes; P1W is a week. A duration is deliberately not a fixed number of seconds, because a month is not a fixed length. An interval pairs two timestamps, or a timestamp and a duration, with a slash: 2026-03-04T09:00:00Z/PT1H is the hour starting at nine. Recurring intervals carry an R prefix with a count.

Years outside 0000 to 9999

Four-digit years cover the practical range, and the standard permits expanded years with a sign and extra digits, such as +012026 or -000050, by agreement. The year before 1 AD is 0000 and 2 BC is −0001, which is the astronomical convention rather than the historian’s. Very little software supports any of this; JavaScript is one of the few places you will meet six-digit years.

RFC 3339 and what the web uses

Internet protocols usually cite RFC 3339 rather than ISO 8601. It is a profile of the same format with the options removed: a full date and time are always required, the offset is mandatory, fractional seconds are permitted, and lowercase t and z are tolerated. It does not allow week dates, ordinal dates, durations or reduced precision. If you need one rule for an API, “RFC 3339 with the offset always written” is it, and 2026-03-04T13:30:00Z satisfies both standards.

Common mistakes

Three recur. Writing a timestamp without an offset and expecting the reader to know the zone; use Z or an explicit offset every time. Using a lowercase mm for month in a format string when the library reads it as minutes, which produces plausible-looking dates with the wrong month. And formatting the year with YYYY in a library where that means the ISO week-based year, which differs from the calendar year for a few days around New Year; use yyyy unless you specifically want week years. The time format converter shows the same instant in several notations side by side, which is the quickest way to catch all three.