ISO 8601: The International Date and Time Format and How to Read It
October 29, 2026
Is 03/04/05 the third of April 2005, the fourth of March 2005, or the fifth of April 2003? Different countries use different orders, which causes errors in everything from airline tickets to software. ISO 8601 is an international standard that avoids the ambiguity by always writing dates from largest unit to smallest: year, month, day. October 30, 2026 is 2026-10-30. Sorting the text alphabetically also sorts the dates chronologically.
The standard also defines times, time zone offsets, week numbers, ordinal dates, durations, and intervals, making it the base for date handling in computing, data exchange, and scientific records.
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A calendar date: YYYY-MM-DD (2026-10-30). Time: hh:mm:ss in 24-hour format (14:30:00). Combined date and time: separated by the letter T (2026-10-30T14:30:00). A trailing Z means UTC: 2026-10-30T14:30:00Z. An offset from UTC is written as +hh:mm or −hh:mm: 2026-10-30T14:30:00+01:00 is 14:30 in a zone one hour ahead of UTC, which is 13:30 UTC. Fractions of a second follow a decimal point: 14:30:00.250. Basic format omits the hyphens and colons: 20261030T143000Z.
Always include the offset or Z when exchanging timestamps between systems or time zones; without it, the time is local and ambiguous.
| Meaning | ISO 8601 example | Notes |
|---|---|---|
| Date | 2026-10-30 | Year-month-day |
| Date and time (local) | 2026-10-30T14:30:00 | No zone given |
| UTC | 2026-10-30T14:30:00Z | Z means zero offset |
| With offset | 2026-10-30T14:30:00+05:30 | India time, UTC+5:30 |
| Week date | 2026-W44-5 | Week 44, day 5 (Friday) |
| Ordinal date | 2026-303 | Day 303 of the year |
| Duration | P1Y2M3DT4H5M6S | 1 yr 2 mo 3 d 4 h 5 min 6 s |
Week and Ordinal Dates
Week dates use YYYY-Www-D, where weeks start on Monday and week 1 is the week with the first Thursday of the year. October 30, 2026 is a Friday in week 44: 2026-W44-5. Ordinal dates use the day of the year: 2026-303. These are used in business planning, manufacturing, and logistics, where weeks and day numbers are standard. Be aware that the first days of January can belong to the last week of the previous year.
Durations and Intervals
A duration begins with P (for period), then years (Y), months (M), and days (D); a T separates the time part: hours (H), minutes (M), seconds (S). PT90M is 90 minutes; P1W is one week; P3DT12H is three and a half days. An interval is two points separated by a slash: 2026-10-30/2026-12-25, or a start and a duration: 2026-10-30/P56D. Repeating intervals begin with R: R5/2026-10-30T09:00:00Z/P1W is five weekly occurrences. These forms are common in calendar software, APIs, and data files.
Why Use It
It removes ambiguity, sorts correctly, is understood by machines and people worldwide, and works in file names (report_2026-10-30.csv). Use it in spreadsheets, databases, logs, and documents shared across countries. Many regions have laws or standards that recommend it. Display dates locally for casual readers if needed, but store and exchange ISO 8601.
Frequently Asked Questions
What is the ISO 8601 date format?
YYYY-MM-DD, for example 2026-10-30.
What does the Z mean in a timestamp?
UTC (zero offset from Coordinated Universal Time).
How is a time zone offset written?
As +hh:mm or −hh:mm after the time, such as +01:00.
What day does an ISO week start on?
Monday.
How do I write a duration of 90 minutes?
PT90M.