Add Hours to a Date in JavaScript

Add Hours to a Date in JavaScript #

To add hours to a Date in JavaScript:

1. Use the getTime() method to get the number of milliseconds since the Unix Epoch.
2. Use the setTime() method, passing it the result of calling getTime plus the hours converted to milliseconds.

Copied!
function addHours(numOfHours, date = new Date()) {
  date.setTime(date.getTime() + numOfHours * 60 * 60 * 1000);

  return date;
}

// 👇️ Add 1 hour to current date
const result = addHours(1);

// 👇️ Add 2 hours to another date
const date = new Date('2022-03-14T09:25:30.820');

// 👇️ Mon Mar 14 2022 11:25:30
console.log(addHours(2, date));

We created a reusable function that takes the number of hours and a Date object and adds the hours to the date.

The getTime method returns the number of milliseconds elapsed between the 1st of January, 1970 00:00:00 and the given date.

We multiplied the number of hours by 60 * 60 * 1000 because we need to convert the value to milliseconds, when passing it to the setTime method.

The setTime() method takes a number that represents the milliseconds since the 1st of January, 1970 and sets the value on the date.

This approach automatically takes care of the scenario where adding X hours pushes us into the next day, month or year.

Copied!
const date = new Date('2022-03-14T23:25:30.820');

const numOfHours = 10;
date.setTime(date.getTime() + numOfHours * 60 * 60 * 1000);

// 👇️ Tue Mar 15 2022 09:25:30
console.log(date);

Adding 10 hours to the date successfully changed the day of the month to the 15th.

Copied!
function addHours(numOfHours, date = new Date()) {
  const dateCopy = new Date(date.getTime());

  dateCopy.setTime(dateCopy.getTime() + numOfHours * 60 * 60 * 1000);

  return dateCopy;
}

// 👇️ Add 2 hours to another date
const date = new Date('2022-03-14T09:25:30.820');

const result = addHours(2, date);

console.log(result); // 👉️ Mon Mar 14 2022 11:25:30

console.log(date); // 👉️ Mon Mar 14 2022 09:25:30 (didn't change original)

We used the getTime method to get the number of milliseconds elapsed between the 1st of January, 1970 00:00:00 and the given date.

Copying the date is quite useful when you have to use the original Date object in other places in your code.

In general, mutating function arguments is a bad practice, as passing the same argument to the same function multiple times returns different results.