# Day.js — Lightweight 2kB Date-Time Library for Modern JavaScript

> A fast, immutable, chainable date library that offers a Moment.js-compatible API in just 2 kilobytes. Supports plugins for advanced formatting, relative time, and timezone handling.

## Install

Save as a script file and run:

# Day.js — Lightweight 2kB Date-Time Library for Modern JavaScript

## Quick Use
```bash
npm install dayjs
```
```js
const dayjs = require('dayjs');
console.log(dayjs().format('YYYY-MM-DD HH:mm:ss'));
console.log(dayjs('2025-01-15').add(7, 'day').format('MMM D, YYYY'));
```

## Introduction
Day.js is a minimalist JavaScript date library that parses, validates, manipulates, and displays dates and times. It was created as a lightweight alternative to Moment.js, offering the same familiar API in a package roughly 60 times smaller. The library is immutable by design, preventing accidental mutation bugs common in date handling code.

## What Day.js Does
- Parses, validates, and formats dates with a chainable API
- Provides locale-aware formatting for 100+ languages
- Supports relative time display (e.g., "3 hours ago")
- Handles timezone conversion and UTC offsets via plugins
- Offers duration, calendar, and advanced formatting through an extensible plugin system

## Architecture Overview
Day.js wraps native JavaScript Date objects in a lightweight immutable wrapper. The core library handles parsing, formatting, and basic manipulation, while all advanced features ship as tree-shakeable plugins. Each operation returns a new Day.js instance rather than mutating the original, making it safe for use in React state and Redux stores.

## Self-Hosting & Configuration
- Install via npm, yarn, or pnpm; also available as a CDN script
- Import only the plugins you need to keep bundle size minimal
- Load locale files for internationalization support
- Works in Node.js, browsers, and serverless environments
- Compatible with ES modules and CommonJS

## Key Features
- Only 2kB minified and gzipped — ideal for performance-critical apps
- API compatible with Moment.js, easing migration
- Immutable operations prevent subtle date mutation bugs
- Plugin architecture keeps the core small while enabling advanced features
- Full TypeScript support with built-in type definitions

## Comparison with Similar Tools
- **Moment.js** — feature-complete but 67kB and mutable; now in maintenance mode
- **date-fns** — functional approach with tree-shaking, but larger API surface
- **Luxon** — built by Moment maintainers with Intl API focus; heavier bundle
- **Temporal (TC39)** — upcoming native standard; not yet widely available

## FAQ
**Q: Can I migrate from Moment.js to Day.js easily?**
A: Yes. Day.js was designed to be a drop-in replacement. Most Moment.js code works with minimal changes.

**Q: Does Day.js handle timezones?**
A: The core library works with local and UTC time. For full IANA timezone support, use the dayjs/plugin/timezone plugin.

**Q: Is Day.js suitable for server-side use?**
A: Absolutely. It runs in Node.js and has no browser-specific dependencies.

**Q: How does the plugin system work?**
A: Plugins are loaded with dayjs.extend(plugin). Each plugin adds methods or formatting tokens to the Day.js prototype without bloating the core.

## Sources
- https://github.com/iamkun/dayjs
- https://day.js.org/

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Author: Script Depot