Fixed the leap year logic in parseIsoDate. The original code used a simple 'year % 4 === 0' check, but the contract requires the proleptic Gregorian calendar rules: February has 29 days when the year is divisible by 400, OR divisible by 4 but not by 100. Changed 'leap' to 'isLeap' with the correct formula: (year % 400 === 0) || (year % 4 === 0 && year % 100 !== 0). npm test now passes.