Day.js
Fast 2KB Moment.js-compatible date library. Immutable date objects with the same Moment.js API — parse, validate, manipulate, and display dates and times. Supports timezones (via plugin), localization (140+ locales), relative time, duration, and date arithmetic. The go-to Moment.js replacement offering identical API at 1/10th the size. Used everywhere Moment.js was used but with better performance and tree-shaking.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Pure computation library — no network calls, no data storage. No security concerns. MIT licensed. Zero dependencies.
⚡ Reliability
Best When
You need Moment.js's API for date manipulation with a dramatically smaller bundle size and immutable objects.
Avoid When
You need functional programming patterns with date-fns, or want the most comprehensive timezone handling (use Luxon or Temporal).
Use Cases
- • Parse, format, and display dates using Moment.js-compatible API without Moment.js's 300KB bundle size
- • Handle timezone-aware date operations using dayjs-plugin-timezone for applications with global users
- • Display relative times ('2 hours ago', '3 days left') using dayjs's relativeTime plugin
- • Perform date arithmetic (add/subtract days, months, years) with chainable, immutable Day.js objects
- • Support 140+ locales for internationalized date formatting using Day.js's locale system
Not For
- • Complex timezone edge cases requiring IANA database precision — date-fns-tz or Luxon may handle edge cases better
- • Temporal API adoption — browser's native Temporal will eventually replace all date libraries; Day.js is a bridge
- • Server-side date manipulation in Rust/Go/Python — Day.js is JavaScript-only
Interface
Authentication
No authentication — utility library.
Pricing
MIT licensed. Completely free. Widely adopted as the Moment.js replacement.
Agent Metadata
Known Gotchas
- ⚠ Plugins must be explicitly loaded — dayjs.extend(relativeTime), dayjs.extend(timezone) etc.; missing extends causes method-not-found errors
- ⚠ Invalid dates silently produce NaN objects instead of throwing — always call .isValid() before using parsed dates
- ⚠ Timezone support requires both utc and timezone plugins together — utc alone doesn't enable tz() method
- ⚠ Locale files must be imported for non-English locales — dayjs.locale() with just a string doesn't work without importing the locale module
- ⚠ Day.js is not compatible with the new TC39 Temporal proposal — don't use Day.js as an abstraction expecting future Temporal compatibility
- ⚠ Mutability: Day.js is immutable but plugins like duration can have mutable behavior — verify plugin documentation
Alternatives
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Scores are editorial opinions as of 2026-03-07.