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---
og_image: "/images/guides/date-fns-vs-dayjs-vs-moment-2026.webp"
title: "date-fns vs Day.js vs Moment.js 2026"
description: "date-fns vs Day.js vs Moment.js 2026: Day.js is 2KB, date-fns is tree-shakeable, Moment is deprecated. Compare bundle size, API design, and timezone support."
date: "2026-04-13"
tier: 2
authors: ["team"]
tags: ["date-fns", "dayjs", "moment", "javascript", "date-library", "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

| | date-fns v4 | Day.js v1 | Moment.js v2 |
|---|---|---|---|
| Weekly Downloads | **~18M** | ~12M | ~14M (legacy) |
| Bundle Size (full) | ~50KB (tree-shakeable) | **~2KB gzipped** | ~294KB minified |
| Tree-Shakeable | **Yes** | Partial (plugins add weight) | No |
| Immutable | **Yes** | **Yes** | No (mutates dates) |
| Timezone Support | date-fns-tz plugin | dayjs/plugin/timezone | moment-timezone (~85KB) |
| TypeScript | **Excellent** | Good | Basic |
| Locale Size | Per-locale import | Bundled (larger) | Per-locale |
| FP module | **Yes** (curried) | No | No |
| Maintained | **Active** | **Active** | Maintenance only |

---

## Why Moment.js Is Off the Table

Moment.js's maintainers themselves recommend against using it in new projects. The [Moment.js documentation](https://momentjs.com/docs/#/-project-status/) 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:

```javascript
// 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.

```typescript
// 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:

```typescript
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:

```javascript
// 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:

```javascript
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:

```javascript
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):

```typescript
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:

```javascript
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:

```bash
# 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)
```

## Decision Checklist for 2026 Teams

| Team situation | Best choice | Why it wins |
|---|---|---|
| New TypeScript app with lots of date calculations | **date-fns** | Pure functions, strong TypeScript ergonomics, no mutable wrapper object, and excellent tree-shaking when imports stay specific. |
| Existing Moment.js app that needs the fastest migration | **Day.js** | The API shape is close enough to Moment that the migration is mostly import and plugin work instead of rethinking every date helper. |
| Tiny marketing site with simple formatting | **Day.js** | The core library stays extremely small, and simple format/parse calls are easy for a mixed-experience team to read. |
| Timezone-heavy scheduling product | **date-fns + date-fns-tz** or **Day.js + timezone plugin** | Pick based on API preference, then write tests around DST transitions. Do not keep Moment solely for timezone support. |
| Legacy codebase that cannot be rewritten this quarter | **Moment.js as a temporary hold** | Freeze new Moment usage, wrap it behind utility functions, and migrate one workflow at a time. |

The main anti-pattern is mixing all three libraries across the same product. If a codebase already has Moment, choose either a Day.js compatibility migration or a date-fns utility-layer migration, then prevent new Moment imports with lint rules or code review.

## Migration Checklist

1. Inventory current imports and split them into formatting, arithmetic, parsing, timezone, and relative-time use cases.
2. Pick the target library per app, not per component. Shared UI packages and app routes should not make different date-library decisions unless bundle isolation is explicit.
3. Add unit tests for DST changes, locale formatting, invalid input, and end-of-month arithmetic before replacing Moment helpers.
4. Replace high-traffic UI formatting first because bundle-size wins show up fastest there.
5. Move scheduling, billing, and audit-log workflows last because they usually carry the most edge cases.

For React date-picker UI decisions, pair this library choice with [React Day Picker vs React Datepicker vs MUI X Date Pickers](/guides/react-day-picker-vs-react-datepicker-vs-mui-x-date-pickers-2026). If your forms validate dates with schemas, also compare [Zod vs Yup vs Joi](/guides/zod-vs-yup-vs-joi-schema-validation-2026).

## Final Verdict

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](/compare/date-fns-vs-dayjs). See also [how to migrate Moment.js to date-fns](/guides/how-to-migrate-momentjs-to-date-fns) and [date-fns v4 vs Temporal API vs Day.js](/guides/date-fns-v4-vs-temporal-api-vs-dayjs-2026).*