ISO 8601 Duration Calculator

An ISO 8601 duration expresses a length of time rather than a point in time, which is why it needs its own grammar. The string opens with P for period, lists date-scale components in fixed order as Y for years, M for months, W for weeks and D for days, then places T before any time-scale components, where H is hours, M is minutes and S is seconds. The T is not decorative: it disambiguates the reused letter M, so P3M is three months while PT3M is three minutes. Zero components are omitted, seconds may carry a decimal fraction, and PT0S is the conventional way to write a zero duration. Typical values are PT30M, PT2H15M30S for a video length, and P1Y3M for a subscription term. The format separates two kinds of quantity that behave differently in arithmetic. Weeks, days, hours, minutes and seconds are exact multiples of one another, but months and years are nominal: a month is 28 to 31 days and a year is 365 or 366, so P1M has no fixed number of seconds. Adding P1M to 31 January is undefined without a resolution rule, and libraries differ on whether the result clamps to 28 February or overflows into March. The conversions shown here use the common approximations of 30 days per month and 365 days per year, which is adequate for display and rough totals but wrong for billing periods, scheduling, or anything crossing a leap year. For exact results, apply the duration to a specific start instant with a calendar-aware library such as java.time or Luxon.

ISO 8601 consolidated several earlier International Organization for Standardization documents on date and time notation, among them ISO 2014 of 1976, which introduced the year-month-day ordering, along with standards covering ordinal dates, week numbering, and time-of-day representation. The combined standard, Data elements and interchange formats - Information interchange - Representation of dates and times, was first published in 1988, with revisions in 1991, 2000 and 2004. In February 2019 it was split into two parts: ISO 8601-1, covering the basic rules including the duration format, and ISO 8601-2, which adds extensions such as uncertain and approximate dates. The motivation throughout was unambiguous international exchange, since national conventions disagree on element order. The duration syntax spread well beyond the standards documents through the specifications that adopted it. W3C XML Schema Part 2, published as a Recommendation in 2001, defined the xsDuration datatype using the same lexical form and made it available to a generation of XML tooling; it also confronted the month ambiguity by defining a partial rather than total order over durations. RFC 3339, published in 2002, profiled ISO 8601 for Internet timestamps and made the format ubiquitous in protocols. Later adopters carried the duration form into everyday web work: schema.org uses it for properties such as cookTime and video duration, so search engines parse it from structured data, and platform APIs return video and subscription lengths in the same notation. Language support followed, including the Duration and Period classes added in Java 8 in 2014.