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Month-end anchored renewal dates: leap-year month length · case 04

Renewals in February of a leap year land on the 28th instead of the 29th.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Input {anchor [y,m,d], n, interval months}. Renewal i (1..n) falls interval*i months after the anchor month, on the anchor day clamped to that month's length (so a 31st anchor returns to the 31st whenever possible). Return the list of [y, m, d].

Why this case matters

Month-end anchors must not drift to the 28th after February, and leap years change the clamp.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression 0	`[[2019, 10, 31], [2019, 12, 31], [2020, 2, 28], [2020, 4, 30], [2020, 6, 30], [2020, 8, 31], [2020, 10, 31], [2020, 12, 31], [2021, 2, 28], [2021, 4, 30], [2021, 6, 30], [2021, 8, 31], [2021, 10, 31]]`	`[[2019, 10, 31], [2019, 12, 31], [2020, 2, 29], [2020, 4, 30], [2020, 6, 30], [2020, 8, 31], [2020, 10, 31], [2020, 12, 31], [2021, 2, 28], [2021, 4, 30], [2021, 6, 30], [2021, 8, 31], [2021, 10, 31]]`	Failed

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