How to 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.

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function addHours(date, hours) {
  date.setTime(date.getTime() + hours * 60 * 60 * 1000);

  return date;
}

// ✅ Add 1 hour to the current date
const result1 = addHours(new Date(), 1);
console.log(result1); // 👉️ 2023-01-13T12:59:22.582Z

// ✅ Add 2 hours to a different date
const date = new Date('2024-03-14T18:30:05.000Z');

const result2 = addHours(date, 2);
console.log(result2); // 👉️ 2024-03-14T20:30:05.000Z

The addHours function takes a Date object and N as parameters and adds N hours to the date.

If you need to add hours to the current date, call the Date() constructor without passing it any parameters.

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function addHours(date, hours) {
  date.setTime(date.getTime() + hours * 60 * 60 * 1000);

  return date;
}

const currentDate = new Date();

// ✅ Add 1 hour to the current date
const result1 = addHours(currentDate, 1);
console.log(result1); // 👉️ 2023-01-13T12:59:22.582Z

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.

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function addHours(date, hours) {
  date.setTime(date.getTime() + hours * 60 * 60 * 1000);

  return date;
}

const date = new Date('2024-03-14T23:30:05.000Z');

const result = addHours(date, 10);
console.log(result); // 👉️ 2024-03-15T09:30:05.000Z

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

# Add Hours to a Date without mutation

The setTime method mutates the Date object it was called on.

If you don't want to change the Date in place, create a copy before calling the method.

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function addHours(date, hours) {
  const dateCopy = new Date(date);

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

  return dateCopy;
}

const date = new Date('2024-03-14T12:30:05.000Z');

const result = addHours(date, 2);
console.log(result); // 👉️ 2024-03-14T14:30:05.000Z

console.log(date); // 👉️ 2024-03-14T12:30:05.000Z

When a Date object is passed to the Date() constructor, it gets converted to a timestamp and can be used to create a copy of the date.

Mutating function parameters is a bad practice because calling the function with the same parameter multiple times returns different results.

Instead, pure functions like the one above return the same output when called with the same parameters.

# Add Hours to a Date using date-fns

You can also use date-fns module to add hours to a date.

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import {addHours} from 'date-fns';

const date = new Date('2024-03-14T12:30:05.000Z');

const result1 = addHours(date, 2);
console.log(result1); // 👉️ 2024-03-14T14:30:05.000Z

const result2 = addHours(date, 4);
console.log(result2); // 👉️ 2024-03-14T16:30:05.000Z

console.log(date); // 👉️ 2024-03-14T12:30:05.000Z

The addHours() function takes a date and the number of hours to be added to the date as parameters.

The function doesn't mutate the original date as shown in the example.

If you don't have date-fns installed, you can install it by running the following command from your terminal.

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# 👇️ create package.json if you don't have one
npm init -y

# ✅ install with NPM
npm install date-fns

# ✅ install with YARN
yarn add date-fns

# Add Hours to a Date using moment.js

You can also use the moment.js module to add hours to a date.

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import moment from 'moment';

const date = new Date('2024-03-14T12:30:05.000Z');

const result1 = moment(date).add(2, 'hours');
console.log(result1); // 👉️ 2024-03-14T09:55:40.000Z

const result2 = moment(date).add(4, 'hours');
console.log(result2); // 👉️ 2024-03-14T09:56:01.000Z

console.log(date); // 👉️ 2024-03-14T12:30:05.000Z

We used the moment().add() method to add hours to a date.

The method can be used to add years, quarters, months, weeks, days, hours, minutes, seconds or milliseconds to a date.

If you don't have moment installed, you can install it by running the following command from your terminal.

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# 👇️ create package.json if you don't have one
npm init -y

# ✅ install with NPM
npm install moment

# ✅ install with YARN
yarn add moment

The call to the add() method actually returns a moment object and not a native JavaScript date.

If you need to convert the value to a JavaScript date, use the toDate() method.

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import moment from 'moment';

const date = new Date('2024-03-14T12:30:05.000Z');

const result1 = moment(date).add(2, 'hours').toDate();
console.log(result1); // 👉️ 2024-03-14T09:55:40.000Z

const result2 = moment(date).add(4, 'hours').toDate();
console.log(result2); // 👉️ 2024-03-14T09:56:01.000Z

console.log(date); // 👉️ 2024-03-14T12:30:05.000Z

The toDate() method takes care of converting the moment object to a native JavaScript Date object.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Add Minutes to a Date in JavaScript
• Add Months to a Date in JavaScript
• How to Add Years to a Date in JavaScript
• How to Add 1 Day to a Date using JavaScript