Validate a Gregorian date

Check that a year, a 1-based month and a day form a real calendar date.

JavaScript Version 1.0.0 · MIT · .md

function isValidDate({ year, month, day }) { const leap = year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0); return day >= 1 && day <= [31, leap ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month - 1]; }

Input & expected output

{
  "year": 2024,
  "month": 2,
  "day": 29
}

true

The edge case

{
  "year": 2026,
  "month": 2,
  "day": 29
}

false

Know the limits

• The original used a zero-based month with new Date(y, m, 0), which reads the previous month’s length and accepted February 30. This version uses a 1-based month and explicit Gregorian rules.
• The lab accepts years 1–9,999 and months 1–12.

The function above is the implementation used by the lab. The lab validates JSON types and caps size before execution; include equivalent validation when accepting untrusted input in your application.

Keep exploring

• Check whether all values are equal — Compare every item with the first using Object.is.
• Check whether an object is empty — Check for own enumerable string keys on a JSON object.
• Check whether an integer is even — Test parity for safe integers, including zero and negative values.