Today's ISO 8601 date is 2026-05-19. In ISO week-date format: 2026-W21-2. This is day 139 of 2026 and ISO week 21. ISO 8601 is humanity's most successful date-format standardization — the format that ended the chaos of MM/DD/YYYY vs DD/MM/YYYY ambiguity and made dates sortable as text, parseable by software, and unambiguous across cultures.
"ISO 8601 chose YYYY-MM-DD because it sorts correctly as a string. That single decision has saved more developer time than any other date format in history."
How we compute this
ISO 8601 is a international standard (gregorian-based) calendar. Each year contains 365 or 366 days; 52 or 53 ISO weeks per year (a year has 53 weeks when it begins on a Thursday or, in leap years, on a Wednesday or Thursday), with each month averaging Same as Gregorian (28–31 days). Years are counted from ISO 8601 standardized in 1988; year-numbering follows the proleptic Gregorian calendar (era: Common Era (no AD/BC distinction in the standard)).
ISO 8601 was first published by the International Organization for Standardization in 1988 (as ISO 8601:1988) and has been revised multiple times since. The standard defines unambiguous notations for: dates (YYYY-MM-DD, like 2026-05-19), times (hh:mm:ss in 24-hour format), datetimes with timezone (YYYY-MM-DDThh:mm:ssZ or with ±hh:mm offset), week dates (YYYY-Www-D, e.g. 2026-W21-1 for Monday of week 21), ordinal dates (YYYY-DDD, day-of-year), intervals (start/end or start/duration), and recurring intervals. The standard was designed to fix the catastrophic ambiguity of national date formats: "03/04/2026" might mean 3 April (UK/Europe) or 4 March (USA) — but "2026-04-03" is unambiguous worldwide. ISO 8601 was rapidly adopted by computing standards (RFC 3339 for internet timestamps, XML Schema date types, JSON conventions), by database systems (SQL DATE and TIMESTAMP types), by scientific publishing (most astronomical and meteorological data), and by international business. Today, ISO 8601 is so universal in technical contexts that it has effectively become a "lingua franca" of dates across every major programming language, operating system, and database system.
Used by: Universal in software, databases, telecommunications, scientific publishing, supply chains, international business, and aviation.
Regions: Global standard, adopted by ISO member bodies in every country.
Frequently asked
What is the ISO date today?
Today's ISO 8601 calendar date is 2026-05-19. In ISO week-date format: 2026-W21-2. In ordinal-date format: 2026-139. All three notations identify the same day unambiguously.
Why is ISO 8601 the international standard?
Three reasons. Unambiguous: a date in YYYY-MM-DD format can't be confused with any other date format, eliminating the catastrophic ambiguity between American MM/DD/YYYY and European DD/MM/YYYY (a difference that has caused real damage in medical records, financial transactions, and international logistics). Sortable: ISO 8601 dates sort correctly as plain strings, with no special parsing needed — "2025-12-31" sorts before "2026-01-01" in any alphabetical sort. Parseable: every major programming language has built-in support for ISO 8601, making cross-system data exchange trivial. The cumulative time savings from this format may be the single largest standardization win in computing history.
What is ISO week 1?
ISO week 1 is defined as the week containing the year's first Thursday — equivalent to the week containing 4 January, or the first week with most of its days (4 or more) falling in the new year. This produces a clean rule for the boundary: week 1 always has 4 January in it, and week 52 (or 53) always has 28 December. Because ISO weeks start on Monday, the early days of January or late days of December may belong to an ISO week of the previous or following year — so 1 January 2027 may be in ISO week 53 of 2026, and 31 December 2026 may be in ISO week 1 of 2027. The ISO standard year and the Gregorian year therefore don't always match for dates near the year boundary.
Is 2026-05-19 the same as 19/05/2026?
The same date, different formats. "2026-05-19" is unambiguous — only one interpretation exists worldwide. "19/05/2026" is unambiguous in countries that use DD/MM/YYYY (UK, Europe, India, Australia) but could be confused with the American convention "05/19/2026" (or with "19 May 2026" written backwards as "19/05/2026"). Worse, "05/06/2026" could be 5 June (DD/MM) or 6 May (MM/DD) — a real-world cause of medical and financial errors. The ISO 8601 format prevents this entirely.
What does the "T" mean in ISO datetimes?
The capital "T" is a literal separator between the date and time portions of an ISO 8601 datetime — for example, "2026-05-19T14:30:00Z" means 19 May 2026 at 14:30:00 UTC. The "T" is required by the strict standard, though many systems accept a space instead. The trailing "Z" stands for "Zulu time" (military code for UTC, the +00:00 timezone offset). For other timezones, ISO 8601 uses explicit offsets: "2026-05-19T14:30:00+03:00" means 14:30 in a timezone three hours ahead of UTC (such as East Africa, Eastern European Summer Time, or Moscow).
When was ISO 8601 created?
The first version was published in 1988 as ISO 8601:1988, drawing on earlier ISO standards from 1971 and 1975 and on the older RFC 2822 internet timestamp format. The standard has been revised several times — most recently as ISO 8601-1:2019 and ISO 8601-2:2019 (which split the standard into two parts, "Basic rules" and "Extensions"). The current version remains backward-compatible with the 1988 original for ordinary calendar and week dates.