FA-77119 / Calendar recurrence rules / Member archive

Yearly ISO BYWEEKNO/BYDAY expansion: week count · case 04

BYWEEKNO=-1 is resolved to week 0 in years whose December 31 belongs to next year's week 1.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

FREQ=YEARLY with BYWEEKNO (ISO weeks, Monday start, week 1 contains January 4) and BYDAY weekdays (0=Monday). Negative week numbers count from the last ISO week of the year (-1 is the last). A year has 53 weeks exactly when December 28 falls in week 53; week numbers outside 1..weeks are ignored for that year. Dates may fall in the adjacent calendar year. Expand years year_start .. year_start+years-1 and return unique ISO dates sorted.

Why this case matters

Recurring calendar series are expanded into concrete instances for display, reminders and conflict checks.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
week 20 monday friday	`["2023-05-15", "2023-05-19"]`	`["2023-05-15", "2023-05-19", "2024-05-13", "2024-05-17"]`	Failed

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