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Trial conversion stub period: previous anchor January wrap · case 02

January trials that end before the anchor day produce a negative stub charge.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input {trial_end date, anchor_day 1..28, price}. If trial_end.day equals anchor_day, charge the full price and the next anchor is one month later. Otherwise charge price*days/cycle half-up, where days runs from trial_end to the next anchor date strictly after it and cycle is the length of the anchor-to-anchor month containing trial_end. Return [amount, next anchor ISO].

Why this case matters

Converting a trial onto a fixed billing anchor produces a stub invoice whose length depends on real month boundaries.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression 0	`[-104, "2025-01-13"]`	`[1123, "2025-01-13"]`	Failed

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