Get the first date in the month of a date

Set the UTC day of the month to 1.

JavaScript Version 1.0.0 · MIT · .md

function firstDateOfMonth(date) { const d = new Date(date); d.setUTCDate(1); return d.toISOString().slice(0, 10); }

Input & expected output

"2026-10-28"

"2026-10-01"

The edge case

"2024-02-29"

"2024-02-01"

Know the limits

• Works on UTC calendar dates; the original built a local Date at midnight.

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.