Date.prototype.setMonth() changes the month (and optionally the day of the month) of a date in local time. Months are zero-indexed (0 = January … 11 = December). It mutates the Date in place and returns the new timestamp.
date.setMonth(month)
date.setMonth(month, day)
0 = January … 11 = December
→ returns new timestamp (ms)
→ mutates date
setMonth(3) // April, keep day
setMonth(3, 1) // April 1
setMonth(1) on Jan 31 → often March (overflow)Call date.setMonth. Start with this line.
date.setMonth(m[, day])| Call | What it does |
|---|---|
date.setMonth(m[, day]) | Primary signature. |
date.setMonth(...) | Call with the arguments your case needs. |
Built into JavaScript. Call setMonth(...) on a Date instance — no import required.
date.setMonth(m[, day])| Parameter | Description |
|---|---|
| monthValue | Required. Local month index (0–11; overflow allowed) |
| dateValue | Optional. Day of month; keep current if omitted |
| Return Value | Description |
|---|---|
| * | New epoch ms (number), not the Date |
Change the month index and keep the existing day of the month.
const event = new Date(1975, 7, 19, 23, 15, 30); // Aug 19
event.setMonth(3); // April
console.log(event.getMonth()); // 3
console.log(event.getDate()); // 197, day 19.setMonth(3). Index 3 means April.19.event now reads month 3.Pass a second argument to set the day of the month in the same call.
const event = new Date(1975, 7, 19);
event.setMonth(3, 1); // April 1
console.log(event.getMonth()); // 3
console.log(event.getDate()); // 119.setMonth(3, 1) means April 1.3, day becomes 1.setDate.Moving Jan 31 into February overflows because February is too short.
const endOfMonth = new Date(2016, 0, 31); // Jan 31, 2016
endOfMonth.setMonth(1); // ask for February
console.log(endOfMonth.getMonth()); // 2 (March)
console.log(endOfMonth.getDate()); // 231 in a leap year.setMonth(1) keeps day 31 conceptually.getMonth() = 2, getDate() = 2).Date.prototype.setMonth() has been in JavaScript since ES1. It is Baseline and works in every modern browser and Node.js — no polyfill needed.
Supported everywhere Date exists: Chrome, Firefox, Safari, Edge, Internet Explorer, and all Node.js versions.
Bottom line: Safe to use in any environment. Focus on zero-based months and end-of-month overflow. See also the MDN setMonth() reference.
Learn how to set the seconds portion of a Date in local time.
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