The ISO 8601 Date and Time Format

ISO 8601 is the international standard for writing dates and times. Its headline rule — dates as YYYY-MM-DD — removes the endless confusion between the American MM/DD/YYYY and the European DD/MM/YYYY orderings, because year-month-day is unambiguous and sorts correctly as plain text.

The date

A date is written largest unit first: 2026-07-14 means the 14th of July, 2026. Because the components run from most to least significant, sorting these strings alphabetically also sorts them chronologically — which is why databases and file names love the format.

The time and time zone

Time is appended after a "T": 2026-07-14T09:30:00. A trailing "Z" means the time is in UTC. An offset like +05:30 or −04:00 states the zone directly, as in 2026-07-14T09:30:00+05:30.

This combination — date, T, time, zone — is what most APIs return, and it is precise enough to pin a single instant anywhere on Earth without ambiguity.

Week dates and durations

ISO 8601 also defines week numbering (2026-W29 for the 29th week) and durations (P1Y2M10D for "1 year, 2 months, 10 days"). These are less common but appear in scheduling systems and project tools.