ISO 8601 Date and Time Format
The international standard for representing dates, times, and datetime values
What is ISO 8601?
ISO 8601 is an international standard published by the International Organization for Standardization that defines formats for representing dates, times, and durations. It provides unambiguous, machine-readable date and time formats that work across different systems and cultures.
The standard was first published in 1988 and has become the de facto format for datetime exchange in computing, international trade, and documentation.
Date Format
The basic ISO 8601 date format is:
YYYY-MM-DDWhere:
- YYYY - 4-digit year (e.g., 2026)
- MM - 2-digit month (01-12)
- DD - 2-digit day (01-31)
Examples:
2026-04-24- April 24, 20262000-01-01- January 1, 20001999-12-31- December 31, 1999
Time Format
The ISO 8601 time format uses 24-hour clock notation:
HH:MM:SSWhere:
- HH - Hours (00-23)
- MM - Minutes (00-59)
- SS - Seconds (00-59)
Examples:
14:30:00- 2:30 PM00:00:00- Midnight23:59:59- One second before midnight
Combined Date and Time
When combining date and time, use the letter T as a separator:
YYYY-MM-DDTHH:MM:SSExamples:
2026-04-24T14:30:00- April 24, 2026 at 2:30 PM2026-01-01T00:00:00- Start of January 1, 2026
Timezone Information
ISO 8601 supports timezone offsets in two ways:
UTC (Zulu Time)
Use Z to indicate UTC:
2026-04-24T14:30:00ZThe Z stands for "Zulu" (military designation for UTC).
Offset from UTC
Specify the offset from UTC using ±HH:MM:
+00:00- UTC (same as Z)+05:30- IST (India Standard Time)-05:00- EST (Eastern Standard Time)+09:00- JST (Japan Standard Time)
Decimal Fractions
You can add decimal fractions for precision:
14:30:00.5- 14:30:00 and 500 milliseconds14:30:00.123456- With microseconds
Why Use ISO 8601?
Benefits of ISO 8601:
- Unambiguous - No confusion between US (MM/DD) and European (DD/MM) formats
- Sortable - Dates sort correctly when treated as strings
- International - Recognized worldwide
- Machine-readable - Easy to parse in any programming language
- Future-proof - Handles years beyond 9999
Related Formats
ISO 8601 is closely related to these standards:
- RFC 3339 - A profile of ISO 8601 used in internet protocols. [RFC 3339]
- Unix timestamps - Seconds since January 1, 1970 (UTC). Learn more
- ISO week dates - Alternative date representation using week numbers. [Wikipedia: ISO 8601]
See Also
- UTC Explained - The global time standard
- Unix Timestamps - Epoch time for developers
- Time Zones Guide - How world time zones work