FA-58462 / Loan amortization schedules / Member archive

Odd first period day count: actual basis · case 02

Actual/360 loans accrue interest on a 365-day year.

Member previewVariant 2 · 3 implementations · 7 checks per implementation

Case contract

x = {'principal', 'rate_bp', 'convention': '30/360'|'act/365'|'act/360', 'start': [y,m,d], 'first_due': [y,m,d]}. 30/360: a start day of 31 becomes 30; then a due day of 31 becomes 30 when the (adjusted) start day is 30; days = 360*dy + 30*dm + dd. Actual conventions count calendar days, basis 365 or 360 (365 even in leap years). If days <= 0 return {'error': 'due_not_after_start'}. Interest = round_half_up(P*bp*days/(10000*basis)). Return {'days', 'basis', 'interest'}.

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: actual basis	`{"basis": 365, "days": 323, "interest": 546}`	`{"basis": 360, "days": 323, "interest": 554}`	Failed

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