FA-58765 / Loan amortization schedules / Member archive

Installment due date calendar: anchor day drift · case 05

After February every due date sticks to the 28th.

Member previewVariant 5 · 3 implementations · 7 checks per implementation

Case contract

x = {'first_due': [y, m, d] (d may exceed the month length), 'count', 'roll': 'none'|'following'|'modified_following', 'holidays': ISO dates}. Installment i falls in month m+i on the anchor day d clamped to the month length (later months return to d). Unless roll is 'none', a Saturday, Sunday or holiday rolls forward to the next business day; under modified_following a roll that leaves the month instead rolls back to the previous business day. Return ISO date strings.

Why this case matters

Amortization engines drive borrower statements, payoff quotes and investor remittances; a misplaced rounding step, boundary or ordering rule compounds across hundreds of periods.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression: anchor day drift	`["2023-10-30", "2023-11-30", "2023-12-30", "2024-01-30", "2024-02-29", "2024-03-29", "2024-04-29", "2024-05-29", "2024-06-29", "2024-07-29", "2024-08-29", "2024-09-29", "2024-10-29"]`	`["2023-10-30", "2023-11-30", "2023-12-30", "2024-01-30", "2024-02-29", "2024-03-30", "2024-04-30", "2024-05-30", "2024-06-30", "2024-07-30", "2024-08-30", "2024-09-30", "2024-10-30"]`	Failed

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