How to Subtract 6 Months From a Date in JavaScript

Tari Ibaba
Last updated on October 11, 2022
How to Subtract 6 Months From a Date in JavaScript

To subtract 6 months from a date:

  1. Call the getMonth() method on the date to get the months.
  2. Subtract 6 from the return value of getMonth().
  3. Pass the result of the subtraction to the setMonth() method.

For example:

function subtract6Months(date) {
  date.setMonth(date.getMonth() - 6);

  return date;
}

// November 14, 2022
const date = new Date('2022-11-14T00:00:00.000Z');

const newDate = subtract6Months(date);

// May 14, 2022
console.log(newDate); // 2022-05-14T00:00:00.000Z

Our subtract6Months() function takes a Date object and the number of months to subtract as arguments. It returns the same Date object with 6 months subtracted.

The Date getMonth() 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, setMonths() 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() - 6);

// July 10, 2021
console.log(date); // 2021-07-10T00:00:00.000Z

In this example, we subtract 6 months from a date in January 2022. This makes the year automatically get rolled back to 2021 by setMonth().

Avoid side-effects

The setMonth() mutates the Date object it is called on. This introduces a side effect into our subtract6Months() 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 subtract6Months(date) {
  // 👇 Make copy with "Date" constructor
  const dateCopy = new Date(date);

  dateCopy.setMonth(dateCopy.getMonth() - 6);

  return dateCopy;
}

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

const newDate = subtract6Months(date);

// February 13, 2022
console.log(newDate); // 2022-02-13T00:00:00.000Z

// 👇 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 6 months from a date. It works like our pure subtractMonths() function. subMonths() takes a Date object and the number of months to subtract as arguments. It returns a new Date object with the months subtracted.

import { subMonths } from 'date-fns';

// July 26, 2022
const date = new Date('2022-07-26T00:00:00.000Z');

const newDate = subMonths(date, 6);

// January 26, 2022
console.log(newDate); // 2022-01-26T00:00:00.000Z

// Original not modified
console.log(date); // 2022-07-26T00:00:00.000Z

Note that the subMonths() function returns a new Date object without modifying the one passed to it.

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