FA-59384 / Subscription proration billing / Member archive

Month-end anchored renewal dates: anchor day drift · case 04

After a short month, renewals stay on the 28th or 30th for the rest of the subscription.

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	`[[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 28], [2026, 4, 28], [2026, 5, 28], [2026, 6, 28]]`	`[[2025, 10, 30], [2025, 11, 30], [2025, 12, 30], [2026, 1, 30], [2026, 2, 28], [2026, 3, 30], [2026, 4, 30], [2026, 5, 30], [2026, 6, 30]]`	Failed

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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