FA-59729 / Subscription proration billing / Member archive

Current billing period containing a date: yearly interval scaling · case 04

Yearly plans roll over monthly and monthly plans yearly.

Member previewVariant 4 · 3 implementations · 9 checks per implementation

Case contract

Input {anchor, interval week|month|year, count, at >= anchor}. Period k starts at anchor + k*count intervals (weeks as 7 days; months/years by calendar month with the anchor day clamped to month length). Return [start ISO, end ISO] of the period with start <= at < end.

Why this case matters

Proration needs the exact period containing a change date; boundary dates belong to the new period.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression 0	`["2024-06-30", "2024-09-30"]`	`["2023-03-31", "2026-03-31"]`	Failed

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