date-fns — Modern Modular JavaScript Date Utility Library

What it is

date-fns is a modern JavaScript date utility library providing over 200 functions for parsing, formatting, arithmetic, and comparison of dates. It is tree-shakeable (import only what you use), immutable (never mutates the input date), and TypeScript-first. It serves as the modern alternative to Moment.js, which is now in maintenance mode.

date-fns is for frontend and backend JavaScript/TypeScript developers who need reliable date manipulation without pulling in a large, monolithic library.

How it saves time or tokens

Native JavaScript Date methods are inconsistent and limited. Formatting a date as 'April 15, 2026' requires manual string concatenation. Calculating 'how many business days between two dates' requires writing custom logic. date-fns provides these as one-line imports.

Because date-fns is tree-shakeable, your production bundle only includes the functions you actually use. A project that imports format and addDays ships only those two functions, not the entire 200+ function library.

How to use

1. Install date-fns:

npm i date-fns

1. Import and use specific functions:

import { format, formatDistanceToNow, addDays, isAfter, parseISO } from 'date-fns';

const now = new Date();

// Format a date
format(now, 'yyyy-MM-dd HH:mm');  // '2026-04-15 14:30'

// Relative time
formatDistanceToNow(parseISO('2026-04-01'), { addSuffix: true });
// 'about 14 days ago'

// Arithmetic
const tomorrow = addDays(now, 1);
isAfter(tomorrow, now);  // true

1. Use with locales for internationalization:

import { format } from 'date-fns';
import { de } from 'date-fns/locale';

format(new Date(), 'PPPP', { locale: de });
// 'Mittwoch, 15. April 2026'

Example

Calculating business days and formatting for a report:

import {
  eachDayOfInterval,
  isWeekend,
  format,
  startOfMonth,
  endOfMonth,
} from 'date-fns';

const start = startOfMonth(new Date());
const end = endOfMonth(new Date());

const businessDays = eachDayOfInterval({ start, end })
  .filter(day => !isWeekend(day));

console.log(`Business days this month: ${businessDays.length}`);
businessDays.forEach(day => {
  console.log(format(day, 'EEE, MMM d'));
});

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Common pitfalls

• date-fns works with native JavaScript Date objects, which are mutable and timezone-unaware. For timezone handling, use the date-fns-tz companion package.
• The format function uses Unicode tokens (yyyy, MM, dd), not Moment.js tokens (YYYY, MM, DD). Migrating from Moment.js requires updating format strings.
• Importing from the root date-fns package in older bundlers may pull the entire library. Use direct imports like import { format } from 'date-fns' with a modern bundler for proper tree shaking.