JavaScript Date getMonth() Method

What You’ll Learn

getMonth() returns the month as a zero-based index from 0 to 11 in local time (January = 0). This guide covers syntax, five examples, month-name mapping, UTC comparisons, and the off-by-one pitfall every beginner hits.

Introduction

Filters, billing cycles, and “posted in March” labels need the month from a Date. The getMonth() method returns that value as an integer—but unlike calendar labels, JavaScript counts months from zero.

Combine it with getFullYear() and getDate() to build custom date strings, or use toLocaleDateString() when locale formatting is enough.

Understanding the getMonth() Method

Date.prototype.getMonth() is a zero-argument instance method. It never mutates the original Date—it only reads the month index according to local time rules.

The zero-based index matches the second argument of new Date(year, monthIndex, day). If you create new Date(2026, 2, 15), getMonth() returns 2 (March).

📝 Syntax

dateObj.getMonth()

Parameters

• None.

Return value

• Integer 0–11 for a valid date in local time.
• NaN if dateObj is an invalid Date.

Month index table

• 0 — January
• 1 — February
• 2 — March
• 3 — April
• 4 — May
• 5 — June
• 6 — July
• 7 — August
• 8 — September
• 9 — October
• 10 — November
• 11 — December

Examples Gallery

Open DevTools Console (F12) or use Try-it links. Fixed dates use new Date(year, monthIndex, day) so indexes stay predictable.

const feb15 = new Date(2026, 1, 15);
const monthIndex = feb15.getMonth();

console.log(monthIndex); // 1 (February)

1

const date = new Date(2026, 1, 15);
const monthNumber = date.getMonth() + 1;

console.log("Calendar month:", monthNumber); // 2 (February)

Calendar month: 2

const MONTHS = [
  "January", "February", "March", "April",
  "May", "June", "July", "August",
  "September", "October", "November", "December"
];

const date = new Date(2026, 1, 15);
console.log("Month:", MONTHS[date.getMonth()]);
// Month: February

Month: February

function safeGetMonth(date) {
  if (!(date instanceof Date) || Number.isNaN(date.getTime())) {
    return null;
  }
  return date.getMonth();
}

console.log(safeGetMonth(new Date(2026, 1, 15)));     // 1
console.log(safeGetMonth(new Date("not-a-date")));     // null

1
null

function isSameMonth(a, b) {
  return a.getFullYear() === b.getFullYear() &&
         a.getMonth() === b.getMonth();
}

const june10 = new Date(2026, 5, 10);
const june28 = new Date(2026, 5, 28);
const nov20 = new Date(2026, 10, 20);

console.log(isSameMonth(june10, june28)); // true
console.log(isSameMonth(june10, nov20));  // false

true
false

🚀 Common Use Cases

• Month filters — show records from “this month.”
• Billing — detect month-end and renewal dates.
• Seasonal UI — swap themes by month index.
• Form defaults — pre-select month in date pickers.
• Analytics — group events by calendar month.
• Date labels — build “Feb 15, 2026” strings.

📝 Notes

• getMonth() is zero-based (January = 0).
• Use getUTCMonth() when you standardize on UTC.
• Invalid dates propagate NaN—validate first.
• Compare with getFullYear() when month spans years matter.
• setMonth(getMonth() + 1) adds one month with automatic year rollover.
• For display, prefer locale formatters over hard-coded English arrays.

💡 Best Practices