FA-60916 / Bond day-count conventions / Open access

Act/Act ISDA year fraction: century years are all treated as leap years · case 01

Pieces in 1900 or 2100 are divided by 366.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The leap test only checks divisibility by four.

VERIFIED REPAIR

Apply the full Gregorian rule including the 100 and 400 exceptions.

Unsuccessful approach: Excluding all century years breaks the year 2000, which is a leap year.

Case contract

Inputs start and end [y,m,d]. If end < start return "end before start". Split the interval at each 1 January; each piece contributes its actual days divided by 366 if the piece lies in a Gregorian leap year, else 365. Sum exactly and return the float rounded to 9 decimals.

Why this case matters

Bond accrual and pricing systems depend on exact day-count arithmetic; a single-day error changes settlement cash.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
from fractions import Fraction
N = 1
observations = []
def solve(a, b):
    A = datetime.date(*a)
    B = datetime.date(*b)
    if B < A:
        return 'end before start'
    def leap(y):
        return y % 4 == 0
    total = Fraction(0)
    cur = A
    while cur < B:
        nxt = min(B, datetime.date(cur.year + 1, 1, 1))
        total += Fraction((nxt - cur).days, 366 if leap(cur.year) else 365)
        cur = nxt
    return round(float(total), 9)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression gregorian century rule 1', [[1900, 12, 12], [1902, 9, 11]], 1.747945205], ['regression gregorian century rule 2', [[2100, 8, 7], [2101, 12, 23]], 1.378082192], ['partial repair probe 1', [[1998, 2, 12], [2003, 1, 12]], 4.915068493], ['partial repair probe 2', [[1997, 7, 29], [2000, 4, 3]], 2.681495621], ['boundary control 1', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[2061, 4, 23], [2062, 11, 8]], 1.545205479], ['normal control 2', [[1965, 4, 28], [1968, 2, 22]], 2.821528558]], [['regression gregorian century rule 1', [[1897, 6, 13], [1901, 8, 19]], 4.183561644], ['regression gregorian century rule 2', [[1900, 4, 25], [1901, 1, 27]], 0.75890411], ['partial repair probe 1', [[2000, 2, 29], [2002, 8, 10]], 2.444277266], ['partial repair probe 2', [[1997, 3, 20], [2001, 12, 29]], 4.778082192], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['normal control 1', [[1982, 10, 1], [1983, 8, 2]], 0.835616438], ['normal control 2', [[1918, 6, 30], [1919, 7, 5]], 1.01369863]], [['regression gregorian century rule 1', [[1899, 7, 30], [1900, 6, 17]], 0.882191781], ['regression gregorian century rule 2', [[2100, 12, 19], [2101, 2, 1]], 0.120547945], ['partial repair probe 1', [[1997, 9, 30], [2002, 8, 23]], 4.895890411], ['partial repair probe 2', [[1997, 5, 12], [2000, 7, 5]], 3.149292612], ['boundary control 1', [[2023, 7, 1], [2024, 7, 1]], 1.001377349], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[2013, 5, 17], [2013, 4, 26]], 'end before start'], ['normal control 2', [[2054, 4, 24], [2054, 11, 1]], 0.523287671]], [['regression gregorian century rule 1', [[1899, 12, 30], [1900, 12, 25]], 0.98630137], ['regression gregorian century rule 2', [[1896, 10, 28], [1900, 5, 19]], 3.55567782], ['partial repair probe 1', [[1997, 4, 13], [2000, 11, 7]], 3.570274721], ['partial repair probe 2', [[2000, 8, 26], [2003, 12, 27]], 3.336028146], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[1970, 8, 5], [1974, 7, 27]], 3.975342466], ['normal control 2', [[1903, 8, 26], [1909, 7, 28]], 5.920547945]], [['regression gregorian century rule 1', [[1898, 8, 12], [1900, 12, 29]], 2.380821918], ['regression gregorian century rule 2', [[2100, 9, 30], [2106, 3, 21]], 5.471232877], ['partial repair probe 1', [[1998, 2, 27], [2002, 8, 30]], 4.504109589], ['partial repair probe 2', [[1999, 1, 19], [2004, 3, 23]], 5.174728647], ['boundary control 1', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[1940, 8, 16], [1943, 2, 14]], 2.497597126], ['normal control 2', [[1986, 3, 5], [1986, 11, 19]], 0.709589041]]]
for label, args, expected in fixtures[N-1]:
    check(label, 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 fixture	Actual	Expected	Outcome
regression gregorian century rule 1	`1.747795494`	`1.747945205`	Failed
regression gregorian century rule 2	`1.37698181`	`1.378082192`	Failed
partial repair probe 1	`4.915068493`	`4.915068493`	Passed
partial repair probe 2	`2.681495621`	`2.681495621`	Passed
boundary control 1	`1.0`	`1.0`	Passed
boundary control 2	`0.0`	`0.0`	Passed
normal control 1	`1.545205479`	`1.545205479`	Passed
normal control 2	`2.821528558`	`2.821528558`	Passed

SHA-256 / 44fe59aa40a6ce1d306282d528c46e932988c7ffb3d31b2a297ce14d951020b9

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
from fractions import Fraction
N = 1
observations = []
def solve(a, b):
    A = datetime.date(*a)
    B = datetime.date(*b)
    if B < A:
        return 'end before start'
    def leap(y):
        return y % 4 == 0 and y % 100 != 0
    total = Fraction(0)
    cur = A
    while cur < B:
        nxt = min(B, datetime.date(cur.year + 1, 1, 1))
        total += Fraction((nxt - cur).days, 366 if leap(cur.year) else 365)
        cur = nxt
    return round(float(total), 9)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression gregorian century rule 1', [[1900, 12, 12], [1902, 9, 11]], 1.747945205], ['regression gregorian century rule 2', [[2100, 8, 7], [2101, 12, 23]], 1.378082192], ['partial repair probe 1', [[1998, 2, 12], [2003, 1, 12]], 4.915068493], ['partial repair probe 2', [[1997, 7, 29], [2000, 4, 3]], 2.681495621], ['boundary control 1', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[2061, 4, 23], [2062, 11, 8]], 1.545205479], ['normal control 2', [[1965, 4, 28], [1968, 2, 22]], 2.821528558]], [['regression gregorian century rule 1', [[1897, 6, 13], [1901, 8, 19]], 4.183561644], ['regression gregorian century rule 2', [[1900, 4, 25], [1901, 1, 27]], 0.75890411], ['partial repair probe 1', [[2000, 2, 29], [2002, 8, 10]], 2.444277266], ['partial repair probe 2', [[1997, 3, 20], [2001, 12, 29]], 4.778082192], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['normal control 1', [[1982, 10, 1], [1983, 8, 2]], 0.835616438], ['normal control 2', [[1918, 6, 30], [1919, 7, 5]], 1.01369863]], [['regression gregorian century rule 1', [[1899, 7, 30], [1900, 6, 17]], 0.882191781], ['regression gregorian century rule 2', [[2100, 12, 19], [2101, 2, 1]], 0.120547945], ['partial repair probe 1', [[1997, 9, 30], [2002, 8, 23]], 4.895890411], ['partial repair probe 2', [[1997, 5, 12], [2000, 7, 5]], 3.149292612], ['boundary control 1', [[2023, 7, 1], [2024, 7, 1]], 1.001377349], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[2013, 5, 17], [2013, 4, 26]], 'end before start'], ['normal control 2', [[2054, 4, 24], [2054, 11, 1]], 0.523287671]], [['regression gregorian century rule 1', [[1899, 12, 30], [1900, 12, 25]], 0.98630137], ['regression gregorian century rule 2', [[1896, 10, 28], [1900, 5, 19]], 3.55567782], ['partial repair probe 1', [[1997, 4, 13], [2000, 11, 7]], 3.570274721], ['partial repair probe 2', [[2000, 8, 26], [2003, 12, 27]], 3.336028146], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[1970, 8, 5], [1974, 7, 27]], 3.975342466], ['normal control 2', [[1903, 8, 26], [1909, 7, 28]], 5.920547945]], [['regression gregorian century rule 1', [[1898, 8, 12], [1900, 12, 29]], 2.380821918], ['regression gregorian century rule 2', [[2100, 9, 30], [2106, 3, 21]], 5.471232877], ['partial repair probe 1', [[1998, 2, 27], [2002, 8, 30]], 4.504109589], ['partial repair probe 2', [[1999, 1, 19], [2004, 3, 23]], 5.174728647], ['boundary control 1', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[1940, 8, 16], [1943, 2, 14]], 2.497597126], ['normal control 2', [[1986, 3, 5], [1986, 11, 19]], 0.709589041]]]
for label, args, expected in fixtures[N-1]:
    check(label, 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 fixture	Actual	Expected	Outcome
regression gregorian century rule 1	`1.747945205`	`1.747945205`	Passed
regression gregorian century rule 2	`1.378082192`	`1.378082192`	Passed
partial repair probe 1	`4.917808219`	`4.915068493`	Failed
partial repair probe 2	`2.682191781`	`2.681495621`	Failed
boundary control 1	`1.0`	`1.0`	Passed
boundary control 2	`0.0`	`0.0`	Passed
normal control 1	`1.545205479`	`1.545205479`	Passed
normal control 2	`2.821528558`	`2.821528558`	Passed

SHA-256 / f7c6c2d814d04cc79ac8ab8ec3cc8747a0e86974c907ae842987975abc7fac36

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import datetime
from fractions import Fraction
N = 1
observations = []
def solve(a, b):
    A = datetime.date(*a)
    B = datetime.date(*b)
    if B < A:
        return 'end before start'
    def leap(y):
        return (y % 4 == 0 and y % 100 != 0) or y % 400 == 0
    total = Fraction(0)
    cur = A
    while cur < B:
        nxt = min(B, datetime.date(cur.year + 1, 1, 1))
        total += Fraction((nxt - cur).days, 366 if leap(cur.year) else 365)
        cur = nxt
    return round(float(total), 9)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression gregorian century rule 1', [[1900, 12, 12], [1902, 9, 11]], 1.747945205], ['regression gregorian century rule 2', [[2100, 8, 7], [2101, 12, 23]], 1.378082192], ['partial repair probe 1', [[1998, 2, 12], [2003, 1, 12]], 4.915068493], ['partial repair probe 2', [[1997, 7, 29], [2000, 4, 3]], 2.681495621], ['boundary control 1', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[2061, 4, 23], [2062, 11, 8]], 1.545205479], ['normal control 2', [[1965, 4, 28], [1968, 2, 22]], 2.821528558]], [['regression gregorian century rule 1', [[1897, 6, 13], [1901, 8, 19]], 4.183561644], ['regression gregorian century rule 2', [[1900, 4, 25], [1901, 1, 27]], 0.75890411], ['partial repair probe 1', [[2000, 2, 29], [2002, 8, 10]], 2.444277266], ['partial repair probe 2', [[1997, 3, 20], [2001, 12, 29]], 4.778082192], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 1, 1], [2025, 1, 1]], 1.0], ['normal control 1', [[1982, 10, 1], [1983, 8, 2]], 0.835616438], ['normal control 2', [[1918, 6, 30], [1919, 7, 5]], 1.01369863]], [['regression gregorian century rule 1', [[1899, 7, 30], [1900, 6, 17]], 0.882191781], ['regression gregorian century rule 2', [[2100, 12, 19], [2101, 2, 1]], 0.120547945], ['partial repair probe 1', [[1997, 9, 30], [2002, 8, 23]], 4.895890411], ['partial repair probe 2', [[1997, 5, 12], [2000, 7, 5]], 3.149292612], ['boundary control 1', [[2023, 7, 1], [2024, 7, 1]], 1.001377349], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[2013, 5, 17], [2013, 4, 26]], 'end before start'], ['normal control 2', [[2054, 4, 24], [2054, 11, 1]], 0.523287671]], [['regression gregorian century rule 1', [[1899, 12, 30], [1900, 12, 25]], 0.98630137], ['regression gregorian century rule 2', [[1896, 10, 28], [1900, 5, 19]], 3.55567782], ['partial repair probe 1', [[1997, 4, 13], [2000, 11, 7]], 3.570274721], ['partial repair probe 2', [[2000, 8, 26], [2003, 12, 27]], 3.336028146], ['boundary control 1', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['boundary control 2', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['normal control 1', [[1970, 8, 5], [1974, 7, 27]], 3.975342466], ['normal control 2', [[1903, 8, 26], [1909, 7, 28]], 5.920547945]], [['regression gregorian century rule 1', [[1898, 8, 12], [1900, 12, 29]], 2.380821918], ['regression gregorian century rule 2', [[2100, 9, 30], [2106, 3, 21]], 5.471232877], ['partial repair probe 1', [[1998, 2, 27], [2002, 8, 30]], 4.504109589], ['partial repair probe 2', [[1999, 1, 19], [2004, 3, 23]], 5.174728647], ['boundary control 1', [[2024, 3, 1], [2024, 3, 1]], 0.0], ['boundary control 2', [[2023, 12, 31], [2024, 1, 1]], 0.002739726], ['normal control 1', [[1940, 8, 16], [1943, 2, 14]], 2.497597126], ['normal control 2', [[1986, 3, 5], [1986, 11, 19]], 0.709589041]]]
for label, args, expected in fixtures[N-1]:
    check(label, 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 fixture	Actual	Expected	Outcome
regression gregorian century rule 1	`1.747945205`	`1.747945205`	Passed
regression gregorian century rule 2	`1.378082192`	`1.378082192`	Passed
partial repair probe 1	`4.915068493`	`4.915068493`	Passed
partial repair probe 2	`2.681495621`	`2.681495621`	Passed
boundary control 1	`1.0`	`1.0`	Passed
boundary control 2	`0.0`	`0.0`	Passed
normal control 1	`1.545205479`	`1.545205479`	Passed
normal control 2	`2.821528558`	`2.821528558`	Passed

SHA-256 / a055fa60a2be860bec4e38b2cff2f35d3ba2c6cffac5d7e3a52c6e0d52242f08

Verification & scope

A deterministic toy contract stated explicitly in the contract field; no claim of conformance to any published convention text. 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 2026-09-29T14:46:50.030303+00:00.

Case digest / b10e58b7a6dfba41afee3f10c9aaec25531878180ac468c691614dbf625c9e76