ISO Week Date Converter

ISO 8601 week dates (YYYY-Www-D) are an alternative calendar where the year contains either 52 or 53 numbered weeks, each starting Monday. Week 1 is the week containing the first Thursday of the year — equivalently, the week containing January 4. Days are 1 (Monday) through 7 (Sunday).

The converter goes both directions between Gregorian dates and ISO week dates, with full handling of 53-week years.

ISO 8601 was first published in 1988 and consolidated several earlier date-format standards. The week date system addresses a recurring problem with the Gregorian calendar: weekly cycles don't align cleanly with year boundaries. The choice of "week 1 contains the first Thursday" is equivalent to several other rules: the week that includes January 4, the week with the majority of its days in the new year, the first week with at least 4 days in the new year. ISO weeks always start on Monday — this is the European/international convention. Some US software treats Sunday as week start with week 1 being whatever week January 1 falls in; that's a different system, often called "week of year" without the ISO label, and produces different week numbers especially around year transitions.

A worked example: December 30, 2024 is a Monday. By ISO 8601, this is 2025-W01-1 — week 1 of 2025, day 1 — because the week containing it has its Thursday (January 2) in 2025. December 31, 2024 is 2025-W01-2. January 5, 2025 is the last day of week 1 (Sunday). January 6, 2025 starts week 2. Conversely, December 30, 2019 was a Monday with Thursday January 2, 2020 — making December 30 the start of 2020-W01. The ISO year 2020 began before the Gregorian year 2020 by 2 days. Most years align cleanly; the boundary cases reveal the system. 53-week years occur when January 1 falls on Thursday or December 31 falls on Thursday in a leap year — recently 2009, 2015, 2020, and 2026 (the current year).

Limitations: ISO weeks don't align with calendar months — week 5 might span late January and early February. Reporting periods that need to align with both months and weeks (most accounting and HR systems) typically use one or the other, not both. The ISO year (Www notation) can differ from the Gregorian year by 1, which is the source of off-by-one errors when programmers use `getYear()` and assume it matches the ISO year. Use the dedicated `getISOYear()` (or equivalent) when working with ISO week dates. The converter handles all edge cases including the 53-week years and the year-boundary mismatches.