What Is ISO 8601?
ISO 8601 is the international standard for writing dates and times. Its basic date format is YYYY-MM-DD, such as 2026-09-27, which avoids confusion between month-first and day-first conventions and sorts correctly as text. Times use 24-hour notation, as in 14:30:00.
Full Timestamps
A combined date and time looks like 2026-09-27T14:30:00Z, where T separates date and time and Z means UTC; a numeric offset such as +05:30 can be used instead.
Weeks and Ordinal Dates
ISO 8601 also defines week dates (2026-W39) and ordinal dates (2026-270).
🧮 Calculate It Yourself
Put ISO 8601 Date Format into practice with these free calculators — no sign-up, nothing stored.
📆
Week Number Calculator
ISO 8601 and US week numbers for any date, and the dates of any week.
Calculate →
📅
Date Calculator
Find days between two dates, or add/subtract days from a date.
Calculate →
🪖
Military Time Converter
Convert between 12-hour and 24-hour (military) time instantly.
Calculate →
🔗 Related Terms
More Time & Dates Terms
- Business Days
- Calendar Quarter
- Chronological Age
- Coordinated Universal Time (UTC)
- Daylight Saving Time (DST)
- Equinox
📖 Browse all glossary terms or explore all free calculators.
Last updated: September 27, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.