Calculate the number of months between two dates

Count calendar month boundaries from start to end.

JavaScript Version 1.0.0 · MIT · .md

function monthDiff({ start, end }) { const a = new Date(start), b = new Date(end); return Math.max(0, (b.getUTCFullYear() - a.getUTCFullYear()) * 12 - a.getUTCMonth() + b.getUTCMonth()); }

Input & expected output

{
  "start": "2026-01-15",
  "end": "2026-10-28"
}

9

The edge case

{
  "start": "2026-10-28",
  "end": "2026-01-01"
}

0

Know the limits

• Days are ignored: January 31 to February 1 counts as one month. An end before the start returns 0.

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.