To subtract 6 months from a date:
- Call the
getMonth()
method on the date to get the months. - Subtract 6 from the return value of
getMonth()
. - 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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