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FA-59731 / Subscription proration billing / Open access

Current billing period containing a date: weekly count multiplier · case 01

Biweekly plans are treated as weekly.

Verified by executionVariant 1 · 9 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The interval count is ignored for weekly plans.

THE FAILURE

The interval count is ignored for weekly plans.

Unsuccessful approach: The attempt adds the count instead of multiplying it.

Case contract

Input {anchor, interval week|month|year, count, at >= anchor}. Period k starts at anchor + k*count intervals (weeks as 7 days; months/years by calendar month with the anchor day clamped to month length). Return [start ISO, end ISO] of the period with start <= at < end.

Why this case matters

Proration needs the exact period containing a change date; boundary dates belong to the new period.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
import calendar
N = 1
observations = []
def solve(x):
    a = datetime.date(*x['anchor'])
    t = datetime.date(*x['at'])
    def shift(k):
        if x['interval'] == 'week':
            return a + datetime.timedelta(days=7 * k)
        months = k * x['count'] * (12 if x['interval'] == 'year' else 1)
        tot = a.month - 1 + months
        y, m = a.year + tot // 12, tot % 12 + 1
        return datetime.date(y, m, min(a.day, calendar.monthrange(y, m)[1]))
    k = 0
    while shift(k + 1) <= t:
        k += 1
    return [shift(k).isoformat(), shift(k + 1).isoformat()]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression (boundary)', {'anchor': [2024, 1, 10], 'interval': 'week', 'count': 2, 'at': [2024, 1, 24]}, ['2024-01-24', '2024-02-07']), ('regression', {'anchor': [2020, 8, 2], 'interval': 'week', 'count': 3, 'at': [2020, 9, 4]}, ['2020-08-23', '2020-09-13']), ('partial-repair probe', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('partial-repair probe', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2020, 10, 31], 'interval': 'year', 'count': 1, 'at': [2022, 2, 15]}, ['2021-10-31', '2022-10-31']), ('normal control', {'anchor': [2023, 10, 31], 'interval': 'month', 'count': 2, 'at': [2024, 2, 3]}, ['2023-12-31', '2024-02-29']), ('normal control', {'anchor': [2022, 10, 31], 'interval': 'month', 'count': 2, 'at': [2023, 6, 30]}, ['2023-06-30', '2023-08-31']), ('normal control', {'anchor': [2023, 5, 19], 'interval': 'year', 'count': 1, 'at': [2025, 8, 18]}, ['2025-05-19', '2026-05-19'])], [('regression', {'anchor': [2020, 8, 2], 'interval': 'week', 'count': 3, 'at': [2020, 9, 4]}, ['2020-08-23', '2020-09-13']), ('regression', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('partial-repair probe', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('partial-repair probe', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2021, 11, 24], 'interval': 'year', 'count': 3, 'at': [2022, 4, 18]}, ['2021-11-24', '2024-11-24']), ('normal control', {'anchor': [2021, 2, 28], 'interval': 'year', 'count': 2, 'at': [2021, 8, 29]}, ['2021-02-28', '2023-02-28']), ('normal control', {'anchor': [2020, 11, 28], 'interval': 'year', 'count': 2, 'at': [2023, 9, 6]}, ['2022-11-28', '2024-11-28']), ('normal control', {'anchor': [2023, 12, 31], 'interval': 'month', 'count': 1, 'at': [2024, 7, 11]}, ['2024-06-30', '2024-07-31'])], [('regression', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('regression', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('partial-repair probe', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('partial-repair probe', {'anchor': [2020, 7, 6], 'interval': 'week', 'count': 2, 'at': [2023, 9, 7]}, ['2023-08-28', '2023-09-11']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2020, 2, 20], 'interval': 'month', 'count': 1, 'at': [2022, 12, 10]}, ['2022-11-20', '2022-12-20']), ('normal control', {'anchor': [2021, 8, 15], 'interval': 'week', 'count': 1, 'at': [2024, 10, 23]}, ['2024-10-20', '2024-10-27']), ('normal control', {'anchor': [2023, 3, 14], 'interval': 'month', 'count': 3, 'at': [2026, 4, 3]}, ['2026-03-14', '2026-06-14']), ('normal control', {'anchor': [2021, 8, 31], 'interval': 'year', 'count': 1, 'at': [2022, 7, 27]}, ['2021-08-31', '2022-08-31'])], [('regression', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('regression', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('partial-repair probe', {'anchor': [2021, 1, 2], 'interval': 'week', 'count': 2, 'at': [2021, 7, 13]}, ['2021-07-03', '2021-07-17']), ('partial-repair probe', {'anchor': [2021, 2, 1], 'interval': 'week', 'count': 2, 'at': [2022, 4, 22]}, ['2022-04-11', '2022-04-25']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2022, 10, 31], 'interval': 'month', 'count': 1, 'at': [2023, 11, 30]}, ['2023-11-30', '2023-12-31']), ('normal control', {'anchor': [2021, 5, 20], 'interval': 'week', 'count': 1, 'at': [2022, 8, 31]}, ['2022-08-25', '2022-09-01']), ('normal control', {'anchor': [2022, 6, 30], 'interval': 'month', 'count': 1, 'at': [2023, 6, 1]}, ['2023-05-30', '2023-06-30']), ('normal control', {'anchor': [2022, 12, 6], 'interval': 'month', 'count': 1, 'at': [2025, 6, 30]}, ['2025-06-06', '2025-07-06'])], [('regression', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('regression', {'anchor': [2020, 7, 6], 'interval': 'week', 'count': 2, 'at': [2023, 9, 7]}, ['2023-08-28', '2023-09-11']), ('partial-repair probe', {'anchor': [2020, 12, 28], 'interval': 'week', 'count': 2, 'at': [2021, 6, 22]}, ['2021-06-14', '2021-06-28']), ('partial-repair probe', {'anchor': [2023, 12, 20], 'interval': 'week', 'count': 3, 'at': [2026, 5, 31]}, ['2026-05-20', '2026-06-10']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2022, 8, 21], 'interval': 'week', 'count': 1, 'at': [2025, 1, 26]}, ['2025-01-26', '2025-02-02']), ('normal control', {'anchor': [2021, 4, 30], 'interval': 'month', 'count': 1, 'at': [2023, 4, 25]}, ['2023-03-30', '2023-04-30']), ('normal control', {'anchor': [2020, 4, 24], 'interval': 'week', 'count': 1, 'at': [2021, 1, 14]}, ['2021-01-08', '2021-01-15']), ('normal control', {'anchor': [2021, 7, 13], 'interval': 'year', 'count': 1, 'at': [2021, 11, 5]}, ['2021-07-13', '2022-07-13'])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression (boundary) 0['2024-01-24', '2024-01-31']['2024-01-24', '2024-02-07']Failed
regression 1['2020-08-30', '2020-09-06']['2020-08-23', '2020-09-13']Failed
partial-repair probe 2['2023-10-27', '2023-11-03']['2023-10-27', '2023-11-10']Failed
partial-repair probe 3['2022-07-09', '2022-07-16']['2022-07-02', '2022-07-16']Failed
boundary control 4['2024-02-29', '2024-03-31']['2024-02-29', '2024-03-31']Passed
normal control 5['2021-10-31', '2022-10-31']['2021-10-31', '2022-10-31']Passed
normal control 6['2023-12-31', '2024-02-29']['2023-12-31', '2024-02-29']Passed
normal control 7['2023-06-30', '2023-08-31']['2023-06-30', '2023-08-31']Passed
normal control 8['2025-05-19', '2026-05-19']['2025-05-19', '2026-05-19']Passed

SHA-256 / d4beef38497d4fec10b29d56f75aaf59aadf6fdc20175294de374269afc2c5a5

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
import calendar
N = 1
observations = []
def solve(x):
    a = datetime.date(*x['anchor'])
    t = datetime.date(*x['at'])
    def shift(k):
        if x['interval'] == 'week':
            return a + datetime.timedelta(days=7 * (x['count'] + k - 1))
        months = k * x['count'] * (12 if x['interval'] == 'year' else 1)
        tot = a.month - 1 + months
        y, m = a.year + tot // 12, tot % 12 + 1
        return datetime.date(y, m, min(a.day, calendar.monthrange(y, m)[1]))
    k = 0
    while shift(k + 1) <= t:
        k += 1
    return [shift(k).isoformat(), shift(k + 1).isoformat()]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('regression (boundary)', {'anchor': [2024, 1, 10], 'interval': 'week', 'count': 2, 'at': [2024, 1, 24]}, ['2024-01-24', '2024-02-07']), ('regression', {'anchor': [2020, 8, 2], 'interval': 'week', 'count': 3, 'at': [2020, 9, 4]}, ['2020-08-23', '2020-09-13']), ('partial-repair probe', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('partial-repair probe', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2020, 10, 31], 'interval': 'year', 'count': 1, 'at': [2022, 2, 15]}, ['2021-10-31', '2022-10-31']), ('normal control', {'anchor': [2023, 10, 31], 'interval': 'month', 'count': 2, 'at': [2024, 2, 3]}, ['2023-12-31', '2024-02-29']), ('normal control', {'anchor': [2022, 10, 31], 'interval': 'month', 'count': 2, 'at': [2023, 6, 30]}, ['2023-06-30', '2023-08-31']), ('normal control', {'anchor': [2023, 5, 19], 'interval': 'year', 'count': 1, 'at': [2025, 8, 18]}, ['2025-05-19', '2026-05-19'])], [('regression', {'anchor': [2020, 8, 2], 'interval': 'week', 'count': 3, 'at': [2020, 9, 4]}, ['2020-08-23', '2020-09-13']), ('regression', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('partial-repair probe', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('partial-repair probe', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2021, 11, 24], 'interval': 'year', 'count': 3, 'at': [2022, 4, 18]}, ['2021-11-24', '2024-11-24']), ('normal control', {'anchor': [2021, 2, 28], 'interval': 'year', 'count': 2, 'at': [2021, 8, 29]}, ['2021-02-28', '2023-02-28']), ('normal control', {'anchor': [2020, 11, 28], 'interval': 'year', 'count': 2, 'at': [2023, 9, 6]}, ['2022-11-28', '2024-11-28']), ('normal control', {'anchor': [2023, 12, 31], 'interval': 'month', 'count': 1, 'at': [2024, 7, 11]}, ['2024-06-30', '2024-07-31'])], [('regression', {'anchor': [2020, 8, 7], 'interval': 'week', 'count': 2, 'at': [2023, 10, 30]}, ['2023-10-27', '2023-11-10']), ('regression', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('partial-repair probe', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('partial-repair probe', {'anchor': [2020, 7, 6], 'interval': 'week', 'count': 2, 'at': [2023, 9, 7]}, ['2023-08-28', '2023-09-11']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2020, 2, 20], 'interval': 'month', 'count': 1, 'at': [2022, 12, 10]}, ['2022-11-20', '2022-12-20']), ('normal control', {'anchor': [2021, 8, 15], 'interval': 'week', 'count': 1, 'at': [2024, 10, 23]}, ['2024-10-20', '2024-10-27']), ('normal control', {'anchor': [2023, 3, 14], 'interval': 'month', 'count': 3, 'at': [2026, 4, 3]}, ['2026-03-14', '2026-06-14']), ('normal control', {'anchor': [2021, 8, 31], 'interval': 'year', 'count': 1, 'at': [2022, 7, 27]}, ['2021-08-31', '2022-08-31'])], [('regression', {'anchor': [2020, 11, 21], 'interval': 'week', 'count': 2, 'at': [2022, 7, 12]}, ['2022-07-02', '2022-07-16']), ('regression', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('partial-repair probe', {'anchor': [2021, 1, 2], 'interval': 'week', 'count': 2, 'at': [2021, 7, 13]}, ['2021-07-03', '2021-07-17']), ('partial-repair probe', {'anchor': [2021, 2, 1], 'interval': 'week', 'count': 2, 'at': [2022, 4, 22]}, ['2022-04-11', '2022-04-25']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2022, 10, 31], 'interval': 'month', 'count': 1, 'at': [2023, 11, 30]}, ['2023-11-30', '2023-12-31']), ('normal control', {'anchor': [2021, 5, 20], 'interval': 'week', 'count': 1, 'at': [2022, 8, 31]}, ['2022-08-25', '2022-09-01']), ('normal control', {'anchor': [2022, 6, 30], 'interval': 'month', 'count': 1, 'at': [2023, 6, 1]}, ['2023-05-30', '2023-06-30']), ('normal control', {'anchor': [2022, 12, 6], 'interval': 'month', 'count': 1, 'at': [2025, 6, 30]}, ['2025-06-06', '2025-07-06'])], [('regression', {'anchor': [2020, 1, 18], 'interval': 'week', 'count': 3, 'at': [2021, 11, 30]}, ['2021-11-20', '2021-12-11']), ('regression', {'anchor': [2020, 7, 6], 'interval': 'week', 'count': 2, 'at': [2023, 9, 7]}, ['2023-08-28', '2023-09-11']), ('partial-repair probe', {'anchor': [2020, 12, 28], 'interval': 'week', 'count': 2, 'at': [2021, 6, 22]}, ['2021-06-14', '2021-06-28']), ('partial-repair probe', {'anchor': [2023, 12, 20], 'interval': 'week', 'count': 3, 'at': [2026, 5, 31]}, ['2026-05-20', '2026-06-10']), ('boundary control', {'anchor': [2024, 1, 31], 'interval': 'month', 'count': 1, 'at': [2024, 2, 29]}, ['2024-02-29', '2024-03-31']), ('normal control', {'anchor': [2022, 8, 21], 'interval': 'week', 'count': 1, 'at': [2025, 1, 26]}, ['2025-01-26', '2025-02-02']), ('normal control', {'anchor': [2021, 4, 30], 'interval': 'month', 'count': 1, 'at': [2023, 4, 25]}, ['2023-03-30', '2023-04-30']), ('normal control', {'anchor': [2020, 4, 24], 'interval': 'week', 'count': 1, 'at': [2021, 1, 14]}, ['2021-01-08', '2021-01-15']), ('normal control', {'anchor': [2021, 7, 13], 'interval': 'year', 'count': 1, 'at': [2021, 11, 5]}, ['2021-07-13', '2022-07-13'])]]
for i, (label, args, expected) in enumerate(fixtures[N-1]):
    check("%s %d" % (label, i), solve(args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
Boundary fixtureActualExpectedOutcome
regression (boundary) 0['2024-01-24', '2024-01-31']['2024-01-24', '2024-02-07']Failed
regression 1['2020-08-30', '2020-09-06']['2020-08-23', '2020-09-13']Failed
partial-repair probe 2['2023-10-27', '2023-11-03']['2023-10-27', '2023-11-10']Failed
partial-repair probe 3['2022-07-09', '2022-07-16']['2022-07-02', '2022-07-16']Failed
boundary control 4['2024-02-29', '2024-03-31']['2024-02-29', '2024-03-31']Passed
normal control 5['2021-10-31', '2022-10-31']['2021-10-31', '2022-10-31']Passed
normal control 6['2023-12-31', '2024-02-29']['2023-12-31', '2024-02-29']Passed
normal control 7['2023-06-30', '2023-08-31']['2023-06-30', '2023-08-31']Passed
normal control 8['2025-05-19', '2026-05-19']['2025-05-19', '2026-05-19']Passed

SHA-256 / 021f46f77a74e3ca11066d39e9bcf1f7abb942141aa0e0e7549be3b4c20b9398

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 9 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

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Verification & scope

A deterministic teaching model of a stipulated billing rule. It makes no claim to reproduce any billing provider's exact behaviour and is not billing software. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:46:39.004886+00:00.

Case digest / d8d425aba35d5312898a085b6bc6e612f7182ef7f650cb9e727338ef0a3f1275