dayjs, js-joda, luxon, and moment are JavaScript libraries designed to simplify working with dates, times, time zones, and internationalization. They address the limitations of JavaScript’s native Date object—such as mutability, poor time zone support, and lack of formatting utilities—by offering immutable APIs, robust parsing, formatting, manipulation, and localization features. While all aim to make temporal logic more predictable and developer-friendly, they differ significantly in architecture, performance characteristics, API design, and long-term viability.
Date Libraries Compared: dayjs, js-joda, luxon, and moment
Working with dates in JavaScript has long been a pain point. The native Date object is mutable, lacks time zone awareness, and offers minimal formatting control. Over the years, libraries like moment, dayjs, luxon, and js-joda emerged to fill this gap—but they take fundamentally different approaches. Let’s break down how they handle real-world scenarios.
Core Philosophy: Mutability vs Immutability
moment uses a mutable model. Calling .add() or .subtract() changes the original object—a frequent source of bugs.
// moment: mutable by default
const now = moment();
const later = now.add(1, 'hour');
console.log(now === later); // true — same object!
In contrast, dayjs, luxon, and js-joda are immutable. Every operation returns a new instance, making code more predictable and easier to debug.
// dayjs: immutable
const now = dayjs();
const later = now.add(1, 'hour');
console.log(now === later); // false — safe!
This shift reflects modern JavaScript best practices, where immutability reduces side effects and simplifies state management in frameworks like React.
Time Zone Handling: Browser Native vs Custom Logic
luxon leans entirely on the browser’s Intl API for time zones. This means no extra data bundles—but also limited control.
// luxon: uses system time zone data
DateTime.now().setZone('America/New_York').toLocaleString();
It works well if your users have up-to-date browsers, but fails silently in older environments or when precise historical DST rules are needed.
dayjs handles time zones via an optional plugin (timezone) that bundles IANA zone data. You get consistent behavior across environments—at the cost of added bytes.
// dayjs with timezone plugin
import timezone from 'dayjs/plugin/timezone';
dayjs.extend(timezone);
dayjs.tz('2023-01-01', 'America/Los_Angeles');
js-joda takes a third path: it ships with its own time zone engine (via @js-joda/timezone). This gives you Java-level precision for recurring events or historical calculations—but increases complexity and bundle size.
moment requires moment-timezone, which bundles the entire tz database. It works but bloats your app.
Parsing Ambiguous Dates: Strictness Matters
How should "01/02/03" be interpreted? American (Jan 2), European (Feb 1), or ISO (2003)?
moment guesses based on locale settings—leading to inconsistent results.
// moment: ambiguous parsing
moment('01/02/03', 'MM/DD/YY'); // assumes US format unless told otherwise
luxon refuses to guess. Without an explicit format, it only accepts ISO 8601 strings.
// luxon: strict by default
DateTime.fromISO('2023-01-02'); // ✅
DateTime.fromFormat('01/02/03', 'MM/dd/yy'); // explicit required
dayjs behaves like moment by default but supports strict parsing via plugins.
js-joda is the strictest: it won’t parse ambiguous strings at all. You must provide a formatter.
// js-joda: always explicit
LocalDateTime.parse('2023-01-02T10:00', DateTimeFormatter.ofPattern('yyyy-MM-dd HH:mm'));
For applications handling user input from multiple regions, explicit parsing prevents subtle data corruption.
Internationalization: Built-in vs Platform-Dependent
luxon shines here. Since it uses Intl, formatting adapts automatically to the user’s locale:
// luxon: locale-aware by default
DateTime.now().setLocale('fr').toLocaleString(DateTime.DATE_FULL);
// → "lundi 2 janvier 2023"
No extra locale files needed—just leverage the browser.
dayjs requires manual locale imports:
import 'dayjs/locale/fr';
dayjs.locale('fr');
js-joda needs companion packages like @js-joda/locale for non-English output.
moment also bundles locales globally, increasing memory footprint.
If your app serves a global audience, luxon’s zero-config i18n is a major advantage.
Performance and Bundle Impact
All modern alternatives beat moment in size and speed, but trade-offs remain:
dayjs: ~2KB core. Plugins add only what you use. Ideal for code-splitting.luxon: ~7KB. Depends on modern browser features; polyfills may be needed.js-joda: ~15KB+ for core, plus time zone data (~50KB+). Heavy but precise.moment: ~70KB with locales—unacceptable for most new projects.
In a React SPA where every kilobyte counts, dayjs often wins. In a dashboard app needing bulletproof date math, js-joda justifies its weight.
Similarities: Shared Strengths Across Libraries
Despite differences, these libraries converge on key modern practices:
1. Immutable Operations
All except moment enforce immutability, aligning with functional programming trends and reducing bugs in reactive UIs.
// Common pattern: chain operations safely
const nextWeek = library.today().plus({ days: 7 }).startOf('day');
2. Fluent APIs
Method chaining improves readability for complex date logic:
// Example across libraries
const deadline = now
.plus({ months: 1 })
.minus({ days: 2 })
.endOf('month');
3. Duration and Interval Support
All offer ways to represent spans of time (not just points):
// luxon
const duration = Duration.fromObject({ hours: 2, minutes: 30 });
// js-joda
const period = Period.ofMonths(1).plusDays(5);
4. Tree-Shaking Friendly (Except moment)
Modern bundlers can eliminate unused code in dayjs, luxon, and js-joda—critical for performance.
Summary: Key Differences
| Feature | dayjs | js-joda | luxon | moment |
|---|---|---|---|---|
| Mutability | Immutable | Immutable | Immutable | Mutable |
| Time Zones | Plugin + bundled data | Full IANA support via addon | Browser Intl API | Plugin + full tz bundle |
| Parsing Strictness | Lenient by default | Strict (explicit formats only) | Strict (ISO or explicit) | Lenient (guesses) |
| i18n | Manual locale loading | Addon required | Automatic via browser | Global locale bundles |
| Bundle Size | Tiny core, modular | Large (but precise) | Moderate (modern browsers only) | Very large |
| New Projects? | ✅ Yes | ✅ For high-precision needs | ✅ For global apps | ❌ Deprecated |
Final Guidance
- Start with
luxonif you’re building a modern web app targeting recent browsers and need strong i18n with minimal setup. - Reach for
dayjsif you’re migrating frommomentor need maximum control over bundle size in a performance-critical app. - Choose
js-jodaonly when you require mathematical rigor—like calculating business days across time zones or handling fiscal calendars. - Avoid
momententirely in new codebases. Its era has passed.
The right choice depends less on features and more on your constraints: browser support, performance budget, team familiarity, and correctness requirements. But one thing is clear: the future of JavaScript date handling is immutable, explicit, and lean.
- dayjs:
Choose dayjs when you need a lightweight, fast, and familiar Moment.js-like API without the legacy baggage. Its plugin architecture lets you include only the functionality you need (e.g., time zones, custom parsing), making it ideal for performance-sensitive applications like SPAs or mobile web experiences where bundle size matters. However, avoid it if you require full ISO 8601 compliance or advanced time zone handling beyond what its plugins provide.
- moment:
Do not use moment in new projects. It is officially deprecated as of September 2020, with maintainers advising against its adoption due to its mutable API, large bundle size, and outdated design. While it remains functional and widely understood, its technical debt (global locales, side-effectful methods) makes it unsuitable for modern, tree-shakeable, or performance-critical applications. Migrate existing codebases to alternatives like dayjs or luxon when feasible.
- luxon:
Choose luxon when you want modern, chainable APIs built on top of the browser’s Intl API for seamless internationalization and time zone support without external dependencies. Its immutable design and clean syntax make it excellent for applications requiring locale-aware formatting (e.g., global SaaS products). Avoid it if you must support older browsers that lack Intl.DateTimeFormat options or if you need to parse ambiguous date strings without explicit formats.
- js-joda:
Choose js-joda when correctness, immutability, and strict adherence to ISO standards are non-negotiable—such as in financial, scientific, or scheduling systems where date math must be deterministic. It’s a faithful port of Java’s battle-tested ThreeTen API, offering rich temporal types like
LocalDateTimeandZonedDateTime. Be prepared for a steeper learning curve and larger bundle size, and note that browser time zone support requires additional configuration.
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Fast 2kB alternative to Moment.js with the same modern API
Day.js is a minimalist JavaScript library that parses, validates, manipulates, and displays dates and times for modern browsers with a largely Moment.js-compatible API. If you use Moment.js, you already know how to use Day.js.
dayjs().startOf('month').add(1, 'day').set('year', 2018).format('YYYY-MM-DD HH:mm:ss');
- 🕒 Familiar Moment.js API & patterns
- 💪 Immutable
- 🔥 Chainable
- 🌐 I18n support
- 📦 2kb mini library
- 👫 All browsers supported
Getting Started
Documentation
You can find more details, API, and other docs on day.js.org website.
Installation
npm install dayjs --save
API
It's easy to use Day.js APIs to parse, validate, manipulate, and display dates and times.
dayjs('2018-08-08') // parse
dayjs().format('{YYYY} MM-DDTHH:mm:ss SSS [Z] A') // display
dayjs().set('month', 3).month() // get & set
dayjs().add(1, 'year') // manipulate
dayjs().isBefore(dayjs()) // query
I18n
Day.js has great support for internationalization.
But none of them will be included in your build unless you use it.
import 'dayjs/locale/es' // load on demand
dayjs.locale('es') // use Spanish locale globally
dayjs('2018-05-05').locale('zh-cn').format() // use Chinese Simplified locale in a specific instance
Plugin
A plugin is an independent module that can be added to Day.js to extend functionality or add new features.
import advancedFormat from 'dayjs/plugin/advancedFormat' // load on demand
dayjs.extend(advancedFormat) // use plugin
dayjs().format('Q Do k kk X x') // more available formats
Sponsors
Support this project by becoming a sponsor. Your logo will show up here with a link to your website.
[Become a sponsor via Github] [Become a sponsor via OpenCollective]
Contributors
This project exists thanks to all the people who contribute.
Please give us a 💖 star 💖 to support us. Thank you.
And thank you to all our backers! 🙏
License
Day.js is licensed under a MIT License.