date-fns vs Day.js vs Moment.js 2026

TL;DR

date-fns for TypeScript projects where tree-shaking matters. Day.js as a 2KB Moment.js drop-in replacement. Moment.js is in maintenance mode — do not start new projects with it. Moment.js is officially deprecated by its maintainers. date-fns (18M weekly downloads) ships hundreds of individual functions — only the ones you import appear in your bundle. Day.js (12M downloads) is immutable, 2KB gzipped, and mirrors Moment's API closely enough to make migration a diff rather than a rewrite.

Quick Comparison

Why Moment.js Is Off the Table

Moment.js's maintainers themselves recommend against using it in new projects. The Moment.js documentation explicitly states this. The technical reasons are fundamental to how Moment was designed:

Mutability is Moment's core problem. Every operation that looks like it creates a new moment actually modifies the existing one:

// Moment — mutable object = hidden bugs
const start = moment('2026-01-01');
const end = start.add(1, 'month'); // MUTATES start!

console.log(start.format('YYYY-MM-DD')); // '2026-02-01' — NOT what you expect
console.log(end.format('YYYY-MM-DD'));   // '2026-02-01'

// To create a new moment, you must clone explicitly:
const end = start.clone().add(1, 'month');
// This is the source of countless subtle bugs in Moment codebases

Bundle size is Moment's second problem. Even if you only use moment.format(), you're shipping 294KB minified, plus ~85KB for timezone support. This is non-negotiable bundle weight for modern frontend applications.

Moment's 14M weekly downloads in 2026 represent existing projects that haven't migrated — not new adoption. Those downloads will decline as teams migrate. Do not start new projects with Moment.

date-fns: Functional, Tree-Shakeable, TypeScript-First

date-fns approaches date manipulation differently: hundreds of pure functions organized by task, each imported individually. You only ship the functions you actually use.

// date-fns — tree-shakeable pure functions
import { format, addMonths, differenceInDays, isAfter, startOfWeek, endOfMonth } from 'date-fns';
import { formatInTimeZone } from 'date-fns-tz';

const now = new Date();

// Formatting
format(now, 'MMMM d, yyyy');                  // 'April 13, 2026'
format(now, 'yyyy-MM-dd HH:mm:ss');           // '2026-04-13 14:30:00'

// Arithmetic — always returns a new Date, never mutates
const nextMonth = addMonths(now, 1);          // New Date object
const daysDiff = differenceInDays(nextMonth, now); // 30

// Comparison
isAfter(nextMonth, now);                      // true

// Calendar utilities
const weekStart = startOfWeek(now, { weekStartsOn: 1 }); // Monday
const monthEnd = endOfMonth(now);

// Timezone-aware formatting
formatInTimeZone(now, 'America/New_York', 'h:mm a zzz'); // '2:30 PM EDT'

date-fns's FP module provides curried versions of every function, enabling functional composition patterns:

import { compose } from 'date-fns/fp';
import { addMonths, format, startOfMonth } from 'date-fns/fp';

// Compose pipeline: get the first day of the month 3 months from now
const formatFirstOfNextQuarter = compose(
  format('MMMM d, yyyy'),
  startOfMonth,
  addMonths(3)
);

formatFirstOfNextQuarter(new Date()); // 'July 1, 2026'

date-fns v4's TypeScript types are precise — function signatures carry full generic type information, and IDE autocomplete for format strings in format() works correctly.

Day.js: The Practical Drop-In Replacement

Day.js was built specifically to replace Moment.js with a compatible API at 2KB. The migration path from Moment to Day.js is the shortest available:

// Moment.js (deprecated)
const date = moment('2026-04-13');
date.format('MMMM D, YYYY');      // 'April 13, 2026'
date.add(1, 'month').format('MM/DD/YYYY'); // '05/13/2026'

// Day.js (drop-in)
const date = dayjs('2026-04-13');
date.format('MMMM D, YYYY');      // 'April 13, 2026'
date.add(1, 'month').format('MM/DD/YYYY'); // '05/13/2026'

Day.js's plugin system extends it without affecting bundle size for users who don't need the plugins:

import dayjs from 'dayjs';
import utc from 'dayjs/plugin/utc';
import timezone from 'dayjs/plugin/timezone';
import relativeTime from 'dayjs/plugin/relativeTime';
import customParseFormat from 'dayjs/plugin/customParseFormat';

dayjs.extend(utc);
dayjs.extend(timezone);
dayjs.extend(relativeTime);
dayjs.extend(customParseFormat);

// Timezone support
dayjs.tz('2026-04-13 14:00', 'America/Los_Angeles')
  .format('YYYY-MM-DD HH:mm z'); // '2026-04-13 14:00 PDT'

// Relative time
dayjs('2026-01-01').fromNow(); // '3 months ago'

// Custom parse format
dayjs('04/13/2026', 'MM/DD/YYYY').format('YYYY-MM-DD'); // '2026-04-13'

Day.js is immutable — unlike Moment, operations return new instances:

const start = dayjs('2026-01-01');
const end = start.add(1, 'month');

start.format('YYYY-MM-DD'); // '2026-01-01' — unchanged
end.format('YYYY-MM-DD');   // '2026-02-01' — new instance

Timezone Handling

Timezone support is where the practical differences between these libraries matter most for production applications.

date-fns-tz handles timezones with IANA timezone names (no DST offset guessing):

import { toZonedTime, fromZonedTime, formatInTimeZone } from 'date-fns-tz';

const utcDate = new Date('2026-04-13T20:00:00Z');
const timezone = 'America/New_York';

// Convert UTC → local timezone representation
const localDate = toZonedTime(utcDate, timezone);
format(localDate, 'h:mm a'); // '4:00 PM'

// Format directly in a timezone
formatInTimeZone(utcDate, timezone, 'MMMM d, yyyy h:mm a zzz');
// 'April 13, 2026 4:00 PM EDT'

// Convert local → UTC
const userInputDate = new Date(2026, 3, 13, 16, 0); // April 13, 4 PM local
const utcEquivalent = fromZonedTime(userInputDate, timezone);

Day.js timezone plugin offers similar capability with Moment-like syntax:

dayjs.tz.guess(); // 'America/New_York' — user's timezone
dayjs().tz('Asia/Tokyo').format('HH:mm'); // Current time in Tokyo
dayjs.tz('2026-04-13 16:00', 'America/New_York').utc().format(); // → UTC

Moment Timezone adds ~85KB for the timezone database. This was acceptable in 2015; it's not acceptable in 2026.

Bundle Size Reality

Bundle size comparison for a typical date utility use case (format + add months + timezone):

date-fns (3 functions + date-fns-tz):  ~12KB gzipped  (tree-shaken)
Day.js + timezone plugin:              ~8KB gzipped
Moment + moment-timezone:              ~380KB gzipped  (no tree-shaking)

For a Next.js app where every KB appears in Lighthouse scores and Core Web Vitals, the 380KB Moment footprint is disqualifying. date-fns and Day.js are both viable — Day.js is smaller for simple formatting cases, date-fns is smaller when you need many specialized functions (date-fns ships only what you import).

When to Use Which

Choose date-fns when:

• TypeScript-first project where type accuracy matters
• You need FP-style composition (date-fns/fp module)
• Tree-shaking is important and you use many different date utilities
• You want a pure-function, no-mutation API with zero global state

Choose Day.js when:

• Migrating from Moment.js and want minimal code changes
• You need the smallest possible bundle for simple formatting/parsing
• Your team finds Moment's chaining API more intuitive than date-fns's functional style

Do not use Moment.js for new projects. If you're maintaining a Moment codebase, plan your migration — both Day.js and date-fns have codemod tools to help:

# Migrate Moment → date-fns
npx moment-to-date-fns

# Or use the Day.js compatibility layer for a faster migration
# (Replace moment() → dayjs() with minimal other changes)

The date library landscape has decisively moved on from Moment.js. The question for new projects is only between date-fns and Day.js — both are actively maintained, immutable, and appropriately sized. date-fns wins on TypeScript accuracy and functional composition; Day.js wins on simplicity and Moment migration path. For existing Moment codebases, Day.js's drop-in compatibility makes it the lowest-friction migration target, while date-fns is the better long-term choice for TypeScript projects building new date utility layers.

Compare date library package stats on PkgPulse. See also how to migrate Moment.js to date-fns and date-fns v4 vs Temporal API vs Day.js.