Get the number of days in a given month

Read day 0 of the next month for a 1-based month.

JavaScript Version 1.0.0 · MIT · .md

function daysInMonth({ year, month }) { const d = new Date(0); d.setUTCFullYear(year, month, 0); return d.getUTCDate(); }

Try in the local lab Download function

Input & expected output

Input

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

Expected output

The edge case

Input

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

Expected output

Know the limits

The original comment called the month zero-based, but new Date(year, month, 0) gives the length of a 1-based month; this page says so explicitly.

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.

Source & maintenance

Uses an explicit 1-based month in UTC; the original comment called the month zero-based although the code used 1-based months. Original MDX retained as inert source evidence.

Maintained by Oveyon. Source commit: 14b481cf60df. Read the license.

Local fixture scope · Node.js 22.23.2 · reviewed October 28, 2026. No external service is required.

Keep exploring

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