JavaScript setMonth(): Set or Change the Month

Date.prototype.setMonth() changes the month of an existing Date in local time. JavaScript months are zero-based—January is 0 and December is 11—and the optional second argument can set the day of the month at the same time.

Syntax:

date.setMonth(monthValue);
date.setMonth(monthValue, dateValue);

For example, month value 3 means April:

const date = new Date(2026, 0, 10);
date.setMonth(3);
console.log(date.toDateString()); // Fri Apr 10 2026

JavaScript Month Numbers Are Zero-Based Top ↑

Setting Month and Day Together Top ↑

The original page used setMonth(3, 5). This sets the month to April and the day to 5:

const date = new Date(2026, 0, 20);
date.setMonth(3, 5);
console.log(date.toDateString()); // Sun Apr 05 2026

Overflow and Negative Month Values Top ↑

Month 12 rolls into January of the next year. Negative months roll into the previous year.

const date = new Date(2026, 5, 5);
date.setMonth(12, 5);
console.log(date.toDateString()); // Tue Jan 05 2027

Adding or Subtracting Months Top ↑

const date = new Date(2026, 8, 10);
date.setMonth(date.getMonth() + 4);
console.log(date.toDateString()); // Sun Jan 10 2027

The End-of-Month Pitfall Top ↑

The current day-of-month participates in the calculation. Changing 31 January directly to February can overflow because February has fewer days.

const date = new Date(2026, 0, 31);
date.setMonth(1);
console.log(date.toDateString()); // rolls into March

If your rule means “same day if possible, otherwise the last valid day,” calculate the destination month’s last day and clamp the original day.

function setMonthClamped(date, monthValue) {{
  const copy = new Date(date);
  const wantedDay = copy.getDate();
  copy.setDate(1);
  copy.setMonth(monthValue);
  const lastDay = new Date(copy.getFullYear(), copy.getMonth() + 1, 0).getDate();
  copy.setDate(Math.min(wantedDay, lastDay));
  return copy;
}}

Return Value and Mutation Top ↑

setMonth() mutates the Date and returns the new timestamp as a number.

Using setUTCMonth() Top ↑

const date = new Date();
date.setUTCMonth(0, 1);
console.log(date.toISOString());

Common setMonth() Mistakes Top ↑

• Using 1 for January instead of 0.
• Ignoring the current day-of-month when switching from a 29th, 30th or 31st to a shorter month.
• Expecting a new Date object rather than mutation.
• Using local setters when the requirement is UTC.

Next: setFullYear() → ← Date Reference

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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