ISO 8601
The international standard for writing dates and times — year first, so dates sort correctly as text.
ISO 8601 is the international standard for writing dates and times, and its core rule is simple: largest unit first. A date written 2026-07-07 goes year, then month, then day, so there’s only one way to read it. That single ordering choice is what makes the standard useful everywhere from database timestamps to shipping labels.
Why year-first wins
Write a date as 07/06/2026 and you’ve created a small ambiguity machine. In the United States that reads as the 6th of July. In Ireland, the UK, and most of the rest of the world, it reads as the 7th of June. Same six digits, two different days, and no way to tell which was meant without knowing the writer’s country. Write it as 2026-07-06 instead, and the ambiguity disappears — every reader, every system, every country parses it the same way.
The year-first structure has a second, quieter benefit: alphabetical order becomes chronological order. Sort a list of ISO 8601 dates as plain text strings, with no special date-parsing logic at all, and they land in correct time order automatically. That’s why the format shows up constantly in filenames, log entries, and spreadsheets that get sorted by a machine rather than read one line at a time by a person.
What else it covers
ISO 8601 extends beyond the date itself. It specifies how to write times alongside a UTC offset, so a timestamp states not just the hour but which time zone it’s anchored to — removing the same kind of ambiguity that plagues date-only formats. It also defines week numbering: weeks run Monday to Sunday, and the standard sets rules for which week a given date belongs to, particularly around the turn of the year, so that “week 1” means the same thing to everyone using it.