dayjs

About

2KB immutable date time library alternative to Moment.js with the same modern API

What detecting dayjs tells you about a site

dayjs reveals a deliberately lightweight date strategy — a 2 KB, Moment-compatible API chosen to avoid Moment's large, mutable footprint. Its presence signals a team that needed friendly date formatting and parsing without paying Moment's bundle cost, and is comfortable wiring up its plugin system.

Why the exact dayjs version matters

Day.js functionality is split into opt-in plugins (utc, timezone, relativeTime); the exact version and which plugins ship tell you how much date capability the page actually carries.

dayjs in a real-world stack

When you find dayjs in a bundle, it rarely travels alone. Often pulled in transitively by UI libraries like Ant Design; otherwise paired directly with the app's date-display components.

Quick facts

How Sourcemap Explorer detects dayjs

dayjs ships as v1.11.21, published 2026-05-26 and carries 0 direct dependencies, 134 versions on the registry. Those exact numbers are the footprint Sourcemap Explorer matches when dayjs rides inside a deployed bundle — here is how the detection works.

We catch dayjs from two complementary signals: bundled source paths and the embedded package.json. Modern bundlers (webpack, Vite, esbuild, Rollup, Turbopack) preserve the original node_modules/dayjs/ paths inside the JavaScript sourcemap's sources[] array — that's the canonical signal. When the matching package.json is also captured in sourcesContent[], we read the exact version field — patch number included. No regex guessing, no version inference.

1. 1

   Confirm the site exposes sourcemaps

   In DevTools Network, check the response headers of any application script for `SourceMap` or `X-SourceMap`. Failing that, fetch the script's last 4 KB and look for a `//# sourceMappingURL=` comment — that map is where the `dayjs` paths live.

2. 2

   Find the package in the bundle

   Open DevTools → Network → reload. Click any application script and look at its sourcemap. Inside, search `sources[]` for entries matching `node_modules/dayjs/` — every match confirms the package is bundled. The matching `sourcesContent[i]` for `node_modules/dayjs/package.json` gives you the exact installed version.

3. 3

   Read the version directly from package.json

   Run `jq -r '. as $m | $m.sources | to_entries[] | select(.value | endswith("node_modules/dayjs/package.json")) | $m.sourcesContent[.key] | fromjson | .version' bundle.js.map`. Sourcemap Explorer automates the same query in the popup.

Recent versions

dayjs README

Live mirror of the GitHub README, for reference. Updated whenever the repo's default branch changes.

English | 简体中文 | 日本語 | Português Brasileiro | 한국어 | Español (España) | Русский | Türkçe | සිංහල

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

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Getting Started

Documentation

You can find for more details, API, and other docs on day.js.org website.

Installation

npm install dayjs --save

📚Installation Guide

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

📚API Reference

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

📚Internationalization

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

📚Plugin List

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.

FAQ