The Leap Day Meltdown — Python Bug Hunt

Briefing

Inspired by the GitHub and Azure outages that both landed on February 29th. Monthly billing anniversaries get pushed "one month forward" — and every date library shortcut breaks on leap years.

billing.py computes the next billing date. Its month table thinks February always has 28 days, and it never clamps.

Bug report

BUG-0229 · Priority: High · Reported by: billing on-call

next_billing_date(year, month, day) -> [year, month, day] rules:

• move one month forward, clamping the day to the target month's length
• February has 29 days in leap years (divisible by 4, except centuries unless divisible by 400)

Observed: Jan 31, 2024 -> "Feb 31" -> exception; Jan 31, 2023 -> Mar 3 drift.

Logs

[billing] anniversary(2024, 1, 31) raised: day is out of range for month

The code as shipped

src/billing.py (editable)

# Computes the next monthly billing anniversary.

DAYS_IN_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

def next_billing_date(year, month, day):
    month = month + 1
    if month > 12:
        month = 1
        year = year + 1
    return [year, month, day]