What Is ISO 8601? The YYYY-MM-DD Date Format, With Examples

ISO 8601 is the International Organization for Standardization (ISO) standard for writing dates and times, and its calendar date is year-month-day, so Friday, Oct. 30, 2026 is written 2026-10-30. ISO’s help center describes it as “a clear and unambiguous way to represent dates and times” (July 31, 2025). This guide writes one moment in every form, explains the trailing Z, and shows where people trip. A one-page cheat sheet to print is at the end.

One moment in every ISO 8601 form

Take Friday, Oct. 30, 2026, at 8:30 a.m. Eastern Daylight Time (EDT). Clocks in the Eastern Time zone are on EDT that day, 4 hours behind Coordinated Universal Time (UTC), so the same instant is 12:30 UTC. The table writes it each way.

Form	Written as	What it says
Calendar date	2026-10-30	Year, month, day, with leading zeros
Ordinal date	2026-303	Day 303 of 2026 (computed)
Week date	2026-W44-5	Week 44, day 5 (Friday; Monday is day 1) (computed)
Date and local time	2026-10-30T08:30:00	Date, a T, then the time; no zone given
Local time with offset	2026-10-30T08:30:00-04:00	8:30 a.m. EDT, 4 hours behind UTC
UTC with Z	2026-10-30T12:30:00Z	The same instant, in UTC
UTC with offset	2026-10-30T12:30:00+00:00	Also UTC; a Z means an offset of 00:00
U.S. style (not ISO)	10/30/2026, 8:30 a.m. EDT	Month first
Day-first style (not ISO)	30/10/2026	Day first

The calendar date, the Z form and the offset form come straight from the standard as ISO’s help center and Request for Comments (RFC) 3339 describe them. The ordinal and week dates are our computation using the rules described by Markus Kuhn’s ISO 8601 summary, hosted by NASA: ordinal day numbers run from 001 to 365 (366 in leap years), and the week starts on Monday.

How does the YYYY-MM-DD format work?

ISO’s help center lists the date as YYYY-MM-DD, year-month-day. The year takes four digits, the month two and the day two, so April 9, 2026 is 2026-04-09, not 2026-4-9. RFC 3339, the internet timestamp profile, spells out the same order: a full-date is the year, a hyphen, the month, a hyphen and the day.

The order runs from the largest unit to the smallest. That has a practical effect you can test: when dates are plain text, an alphabetical sort puts them in date order.

Date	ISO order	U.S. order
Dec. 31, 2025	2025-12-31	12/31/2025
Feb. 15, 2026	2026-02-15	02/15/2026
Oct. 3, 2026	2026-10-03	10/03/2026
Alphabetical sort gives	Dec. 31, 2025, Feb. 15, 2026, Oct. 3, 2026 (correct)	Feb. 15, 2026, Oct. 3, 2026, Dec. 31, 2025 (wrong)

We ran that sort ourselves. It shows a property of the order, not a promise about any particular app, since software can still read the text however it was built to.

What is the difference between 05/01/2026 and 2026-05-01?

The slash version has two readings. Month first, 05/01/2026 is May 1; day first, it is Jan. 5. A reader cannot tell which from the text. 2026-05-01 has one reading, May 1, 2026, because a four-digit number in the first position can only be the year. ISO’s help center calls the standard “unambiguous”; this is the case it means.

What does the Z mean, and how is it different from an offset?

A Z suffix marks UTC. ISO’s help center gives 2022-09-06T18:00:00Z as its example, and RFC 3339 defines Z as “a suffix which, when applied to a time, denotes a UTC offset of 00:00.”

An offset such as -04:00 or +03:00 keeps the local clock reading and says how far it sits from UTC. Both forms name the same instant, so a program can convert between them:

• 2026-10-30T08:30:00-04:00 is 8:30 a.m. on the wall clock in Eastern Daylight Time.
• 2026-10-30T12:30:00Z is that moment in UTC.
• 2026-10-30T15:00:00Z is a different moment: 15:00 UTC, which is 11:00 a.m. EDT. Reading a Z time as local time shifts it by the zone’s offset.

A time with no Z and no offset, such as 2026-10-30T08:30:00, says nothing about the zone. Anyone receiving it has to know the zone from somewhere else.

What are ISO week dates and ordinal dates?

A week date names the year, the week and the weekday: 2026-W44-5 is Friday of week 44. Kuhn’s summary gives the rules: a week starts on Monday (day 1) and ends on Sunday (day 7), and week 01 is the first week that has the Thursday in the year. Google Docs Editors Help states the same for its ISOWEEKNUM function: weeks begin on Monday, and week 1 is the week containing the first Thursday of the year.

Our full walk-through is in ISO week numbers explained. The point here is one trap. The week-year can differ from the calendar year near Jan. 1. By our computation, Jan. 1, 2027 is a Friday and falls in week 53 of 2026, so its week date is 2027-01-01 as a calendar date and 2026-W53-5 as a week date.

An ordinal date counts the days since Jan. 1: 2026-303 is Oct. 30. The method for any date is in how to calculate the day of the year. The Monday start is also why calendar settings matter; see Monday-start vs. Sunday-start calendars if your printout and your app disagree. We keep a Monday-start printable calendar for that case.

Which ISO 8601 forms should you avoid or leave out?

ISO 8601 also has a “basic” format with no hyphens or colons. ISO’s help center advises avoiding it in plain text to prevent ambiguity. The Library of Congress’s Extended Date/Time Format (EDTF) specification, a profile of ISO 8601, says the basic format “is not permitted” in EDTF. For anything people read, use the extended form with hyphens, as in the table above.

The standard is published in two parts, ISO 8601-1 and ISO 8601-2, both dated 2019, according to the Estonian Centre for Standardisation’s listing. ISO’s own pages blocked our automated check, so this guide does not quote the standard’s text beyond what ISO’s help center, RFC 3339 and the secondary summaries above say. For formal wording, such as time intervals, buy or read the standard itself.

How does RFC 3339 relate to ISO 8601?

RFC 3339, “Date and Time on the Internet: Timestamps,” was published by the Internet Engineering Task Force (IETF) in July 2002. It calls itself “a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar.” Its full-date is the same year-month-day order, and its Z is the same UTC marker, so a timestamp like 2026-10-30T12:30:00Z fits both. RFC 3339 has since been updated by RFC 9557.

Common mistakes with ISO 8601 dates

1. Swapping month and day. 2026-30-10 is not a date. In ISO order the month always comes second.
2. Dropping the leading zero. 2026-4-9 does not match YYYY-MM-DD; write 2026-04-09.
3. Treating Z as local time. 15:00:00Z is 15:00 UTC, not 3 p.m. on the reader’s clock.
4. Sending a time with no zone. 2026-10-30T08:30:00 leaves the reader guessing the offset.
5. Mixing the week-year with the calendar year. 2026-W53-5 is Jan. 1, 2027.
6. Assuming every calendar is covered. RFC 3339 states it uses the Gregorian calendar; a date in another calendar system needs its own conversion first.

ISO 8601 cheat sheet to print

The free one-page sheet repeats the table above, the ambiguous-versus-unambiguous pair and three checks to run before you share a timestamp. For a printable year view with week rows, use the printable yearly calendar, or the November printable calendar for the next month.