Get the day of the year from a date

Count days from January 1, which is day 1.

JavaScript Version 1.0.0 · MIT · .md

function dayOfYear(date) { return Math.round((Date.parse(date) - Date.parse(`${date.slice(0, 4)}-01-01`)) / 86400000) + 1; }

Input & expected output

"2026-10-28"

301

The edge case

"2024-12-31"

366

Know the limits

• UTC dates, so daylight-saving changes cannot shift the count; the original subtracted local Date objects.

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

• Compare two dates — Compare validated ISO calendar dates in UTC and return -1, 0 or 1.
• Add AM PM suffix to an hour — Turn an hour from 0 to 23 into a 12-hour label with am or pm.
• Calculate the number of days between two dates — Count whole calendar days between two YYYY-MM-DD dates.