Get the total number of days in a year

Return 366 for Gregorian leap years and 365 otherwise.

JavaScript Version 1.0.0 · MIT · .md

function daysInYear(year) { return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0 ? 366 : 365; }

Try in the local lab Download function

Input & expected output

Input

Expected output

The edge case

Input

Expected output

Know the limits

Proleptic Gregorian rules: 1900 is not a leap year, 2000 is.

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

Keeps the arithmetic variant. 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.

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