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Week-year ≠ calendar year
The ISO week-numbering year can differ from the calendar year around January 1 — 2027-01-01 belongs to 2026-W53, and 2026-12-31 can fall in the next year's W01. Carrying a week number without its week-year is a bug.
Class
Case to account for
Affects
1 conversion
01
Detection
Date is in late December or early January and the week is 52/53/01.
02
Mitigation
01
Always carry (isoYear, isoWeek) as a pair; never a bare week number.
03
Affected conversions
Instant(ISO week × slot) key
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