How to Subtract Months From a Date in JavaScript

1. Date getMonth() and setMonth() methods

To subtract months from a date in JavaScript:

1. Call the getMonth() method on the Date to get the months.
2. Subtract the months.
3. Pass the result of the subtraction to the setMonth() method.

For example:

function subtractMonths(date, months) {
  date.setMonth(date.getMonth() - months);
  return date;
}

// August 13, 2022
const date = new Date('2022-08-13T00:00:00.000Z');

const newDate = subtractMonths(date, 3);

// May 13, 2022
console.log(newDate); // 2022-05-13T00:00:00.000Z

Our subtractMonths() function takes a Date object and the number of months to subtract as arguments. It returns the same Date object with the months subtracted.

The Date getMonth() method returns a zero-based number that represents the month of a particular date.

The Date setMonth() method sets the months of a date to a specified zero-based number.

Note: “Zero-based” here means that 0 is January, 1 is February, 2 is March, etc.

If the months subtracted would decrease the year of the date, setMonth() will automatically update the date information to reflect this.

// January 10, 2022
const date = new Date('2022-01-10T00:00:00.000Z');

date.setMonth(date.getMonth() - 2);

// November 10, 2021: year decreased by 1
console.log(date); // 2021-11-10T00:00:00.000Z

In this example, we subtracted 2 months from a date in January 2022. This makes setMonth() automatically roll the year back to 2021.

Avoid side-effects

The setMonth() method mutates the Date object it is called on. This introduces a side effect into our subtractMonths() function. To avoid modifying the passed Date and create a pure function, make a copy of the Date and call setMonth() on this copy, instead of the original.

function subtractMonths(date, months) {
  // 👇 Make copy with "Date" constructor
  const dateCopy = new Date(date);

  dateCopy.setMonth(dateCopy.getMonth() - months);

  return dateCopy;
}

// August 13, 2022
const date = new Date('2022-08-13T00:00:00.000Z');

const newDate = subtractMonths(date, 3);

// May 13, 2022
console.log(newDate); // 2022-05-13T00:00:00.000Z

// 👇 Original not modified
console.log(date); // 2022-08-13T00:00:00.000Z

Tip: Functions that don’t modify external state (i.e., pure functions) tend to be more predictable and easier to reason about, as they always give the same output for a particular input. This makes it a good practice to limit the number of side effects in your code.

2. date-fns subMonths() function

Alternatively, we can use the subMonths() function from the date-fns NPM package to quickly subtract months from a date. It works like our pure subtractMonths() function.

import { subMonths } from 'date-fns';

// June 27, 2022
const date = new Date('2022-06-27T00:00:00.000Z');

const newDate = subMonths(date, 4);

// February 27, 2022
console.log(newDate); // 2022-02-27T00:00:00.000Z

// 👇 Original not modified
console.log(date); // 2022-06-27T00:00:00.000Z

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