How to Add Weeks to a Date in JavaScript

In this article, we’ll learn how to easily add any number of weeks to a Date object in JavaScript.

1. Date setDate() and getDate() methods

To add weeks to a Date in JavaScript:

1. Use the getDate() method on the Date to get the day of the month of the Date.
2. Add the result of getDate() to 7 multiplied by the number of weeks to add.
3. Call the setDate() method with this sum as an argument.

For example:

function addWeeks(date, weeks) {
  date.setDate(date.getDate() + 7 * weeks);

  return date;
}

// May 20, 2022
const date = new Date('2022-05-20T00:00:00.000Z');

const newDate = addWeeks(date, 1);

// May 27, 2022
console.log(newDate); // 2022-05-27T00:00:00.000Z

Our addDays() function takes a Date object and the number of days to add as arguments, and returns the same Date object with the newly added days.

The Date getDate() method returns a number between 1 and 31 that represents the day of the month of the particular date.

The Date setDate() method changes the day of the month of the Date object to the number passed as an argument.

If the number you specify would change the month or year of the Date, setDate() automatically updates the Date information to reflect this.

function addWeeks(date, weeks) {
  date.setDate(date.getDate() + 7 * weeks);

  return date;
}

// May 20, 2022
const date = new Date('2022-05-20T00:00:00.000Z');

const newDate = addWeeks(date, 3);

// June 10, 2022
console.log(newDate); // 2022-06-10T00:00:00.000Z

Adding 3 weeks adds 21 days (20 + 21 = 41 days), but May has only 31 days, so the month is incremented by 1 and the day is set to 10 (41 – 31).

Avoid side effects

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

function addWeeks(date, weeks) {
  const dateCopy = new Date(date);

  dateCopy.setDate(dateCopy.getDate() + 7 * weeks);

  return dateCopy;
}

const date = new Date('2022-05-20T00:00:00.000Z');

const newDate = addWeeks(date, 3);

console.log(newDate); // 2022-06-10T00:00:00.000Z

// Original not modified
console.log(date); // 2022-05-20T00:00:00.000Z

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 addWeeks() function

Alternatively, you can use the pure addWeeks() function from the date-fns NPM package to quickly add weeks to a Date. It works similarly to our addWeeks() function.

import { addWeeks } from 'date-fns';

const date = new Date('2022-05-20T00:00:00.000Z');

const newDate = addWeeks(date, 3);

console.log(newDate); // 2022-06-10T00:00:00.000Z

// Original not modified
console.log(date); // 2022-05-20T00:00:00.000Z

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