ISO 8601 week number ⇔ date — find week-of-year & week start
Convert between ISO 8601 week numbers (e.g. 2026-W21-6) and calendar dates (e.g. 2026-05-23). Pick the output format with a mode toggle. Week-only input (2026-W21) is parsed as the Monday of that week. Follows ISO rules: W01 is the week containing 4 Jan, weeks start on Monday, and year-boundary cases (2024-12-30 = 2025-W01-1) are handled correctly. Runs entirely in your browser.
How to use
Paste one entry per line, pick the output format (ISO week or date), and click Convert. Week numbers follow ISO 8601: W01 is the week containing 4 January, weeks start on Monday (Day 1) and end on Sunday (Day 7). Year-boundary cases like `2024-12-30` (Mon) → `2025-W01-1` are handled correctly. Week-only inputs (`2026-W21`) are interpreted as that week's Monday. Unparseable lines are skipped and counted in the stats.
FAQ
- Is my input uploaded?
- No. All conversion runs in your browser using plain JavaScript Date math.
- How are ISO 8601 week numbers defined?
- W01 is the week that contains **4 January** (equivalently, the week containing the first Thursday of the year). Weeks start on Monday (Day 1) and end on Sunday (Day 7). Around year boundaries the ISO year and calendar year can differ — e.g. 2024-12-30 (Mon) is 2025-W01-1.
- What does a week-only input (e.g. 2026-W21) convert to?
- The **Monday (Day 1)** of that week. So `2026-W21` → `2026-05-18`, while `2026-W21-6` → `2026-05-23` (Saturday).
- Are there years with 53 weeks?
- Yes. A year has 52 or 53 ISO weeks. 53-week years occur when 1 Jan is a Thursday (or 1 Jan is Wednesday in a leap year). 2026 has **W53**, for example.
- What happens to invalid lines like `2026-W99` or `2026-13-01`?
- They are skipped (not included in the output). The stats reports the skipped count. Anything outside week 1–53, day 1–7, month 1–12, or actual calendar days is rejected.
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