ISO 8601 — International standard for representing dates and times

Basic principles and common forms

The standard provides several related notations. The most familiar is the calendar date in year–month–day order, written in extended form with hyphens as YYYY-MM-DD, for example 2022-11-10. A corresponding time of day is written in hours, minutes and seconds as hh:mm:ss, for example 17:36:00. To combine date and time, a capital letter T separates them: 2022-11-10T17:36:00. A UTC time may be indicated with the letter Z, and local times can include numeric offsets such as +02:00.

Variants and special formats

• Extended vs basic: Extended forms use separators (hyphens and colons) for readability; basic forms omit them (YYYYMMDD, hhmmss).
• Ordinal dates: A year with a day-of-year count, written as YYYY-DDD (e.g., 2022-314 for the 314th day).
• Week dates: Year with ISO week number and weekday as YYYY-Www-D (e.g., 2022-W45-4), used in business planning and production calendars.
• Reduced precision: The standard allows truncated forms like YYYY-MM or just YYYY when full precision is unnecessary.

History and development

ISO 8601 was developed to avoid ambiguity in international communication and to provide a canonical ordering for date-time strings. It was first published by the International Organization for Standardization and later revised and updated to cover additional scenarios such as week numbering, time zone offsets and fractional seconds. Its adoption has grown with computing and networked systems that exchange time-stamped information.

Practical importance and uses

Because ISO 8601 strings sort lexically in chronological order, they are ideal for filenames, log entries, database keys and APIs where consistent ordering matters. Many programming languages and libraries provide parsing and formatting routines for the standard. It is also commonly used in standards for data interchange (for example, in XML, JSON timestamps and many internet protocols).

Adoption and considerations

Although ISO 8601 is widely used in computing and formal documentation, everyday human conventions for writing dates vary by region. Some countries and cultures use day–month–year or month–day–year in daily life, while many East Asian systems follow year–month–day, which coincides with ISO ordering. See the note on sorting and lexical order via sorting behavior and regional practice at regional date formats.

When working with ISO 8601, be explicit about time zones and precision. Use extended notation for readability unless a compact form is required, and prefer including the time zone indicator (Z or an offset) when timestamps cross regions. These simple practices make date-time data portable, unambiguous and easier to automate.