ISO 8601 date format explained (YYYY-MM-DD and beyond)

Is 03/10/2026 the 3rd of October or the 10th of March? It depends on who wrote it. ISO 8601, the international standard for dates and times, removes the guesswork: 2026-10-03 can only mean one day. It is used in data files, APIs, databases and file names around the world.

Dates

• 2026-10-03: year, month, day, always four-two-two digits, separated by hyphens.
• 2026-10: a month.
• 2026-W40: ISO week 40 of 2026; 2026-W40-6 is its Saturday.
• 2026-276: day 276 of the year, an ordinal date, which is the same 3 October.

Times and time zones

• 14:30:00: 24-hour clock, hours, minutes, seconds.
• 2026-10-03T14:30:00: date and time joined by T. Without a zone, this is local time somewhere unspecified, which is ambiguous for anything shared.
• 2026-10-03T14:30:00Z: Z means UTC.
• 2026-10-03T20:15:00+05:45: an offset from UTC, here Nepal time. It is the same moment as 14:30 UTC.
• 2026-10-03T14:30:00.250Z: fractions of a second after a decimal point.

The Timestamp Converter shows any Unix timestamp in ISO 8601 form, both in UTC and with your own offset, along with the day of the year. Pick a date and time and it gives the timestamp and ISO form the other way round.

Durations

ISO 8601 also writes lengths of time, starting with P for period: P3D is three days, PT2H30M is two and a half hours, and P1Y2M is one year and two months. The T separates date parts from time parts, which is why P1M is a month but PT1M is a minute. You meet these in video lengths, schema markup and calendar data.

Why it is the safest format

• Unambiguous in every country: nobody reads 2026-10-03 as March.
• Sorts correctly as plain text, because the largest unit comes first. That makes it ideal for file names; see naming files so they sort.
• Machine-readable: every major programming language and database can parse it.

ISO weeks

ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. So the first days of January can belong to the last week of the previous year, and the ISO week-year can differ from the calendar year around New Year. Week numbers in some countries' calendars, and in some spreadsheet functions, follow different rules, so state which system you mean.

In spreadsheets

Excel and Google Sheets store dates as numbers and display them in your regional format. To write a date as ISO 8601 text, use =TEXT(A2,"yyyy-mm-dd"). To keep dates in that form when typing, set the column's number format to yyyy-mm-dd. Exported CSV files are safest with ISO dates, because every program reads them the same way.

Common mistakes

• Leaving out the zone or offset on times shared between systems.
• Writing single-digit months or days, such as 2026-1-5. Use 2026-01-05.
• Using a space instead of T. Many systems accept it, but strict parsers do not.
• Confusing the offset sign: +05:45 means ahead of UTC.

For time zone abbreviations, see EST vs EDT, GMT vs UTC. For planning across zones, use the Time Zone Meeting Planner.