Day.js

1.11.20 · active · verified Sat Apr 18

Day.js is a minimalist JavaScript library providing a Moment.js-compatible API for parsing, validating, manipulating, and displaying dates and times. At only 2KB, it offers immutability, chainable methods, and extensive internationalization and plugin support. The current stable version is 1.11.20, and the library is actively maintained with frequent bug fix releases.

Common errors

```
ReferenceError: dayjs is not defined
```
cause Attempting to use `dayjs` without importing it first.

fix Add `import dayjs from 'dayjs';` at the top of your file.
```
TypeError: dayjs(...).isBetween is not a function
```
cause Attempting to use a plugin-provided method (like `isBetween`, `utc`, `duration`) without importing and extending its respective plugin.

fix Import the specific plugin and extend Day.js with it: `import isBetween from 'dayjs/plugin/isBetween'; dayjs.extend(isBetween);`
```
Invalid Date
```
cause Day.js could not parse the provided date string into a valid date object, especially for non-standard formats.

fix Ensure the date string is in a standard format (ISO 8601, JavaScript Date string), or use the `CustomParseFormat` plugin for specific formats: `dayjs.extend(customParseFormat); dayjs('25/12/1995', 'DD/MM/YYYY');`
```
dayjs is not a function
```
cause Incorrectly importing Day.js (e.g., `import { dayjs } from 'dayjs'` instead of `import dayjs from 'dayjs'`).

fix Ensure you are using the default import for the main Day.js function: `import dayjs from 'dayjs';`
```
Locale not applying to dates
```
cause The desired locale was imported but not activated globally or for the specific Day.js instance.

fix After importing the locale file (e.g., `import 'dayjs/locale/fr';`), activate it using `dayjs.locale('fr');` for global effect or `.locale('fr')` on an instance for local effect.

Warnings

gotcha Many common date manipulation and query functions (e.g., `isBetween`, `utc`, `duration`, `advancedFormat`) are not included in the core library. They must be explicitly imported and extended as plugins.
Fix: Import the required plugin and extend Day.js: `import isBetween from 'dayjs/plugin/isBetween'; dayjs.extend(isBetween);`
gotcha Day.js only includes the `en` locale by default. To use other locales, you must import them separately and then activate them globally or per instance.
Fix: Import the locale: `import 'dayjs/locale/es';` then activate: `dayjs.locale('es');`
gotcha Day.js instances are immutable. Methods like `add()`, `subtract()`, `set()`, `startOf()`, `endOf()` return a *new* Day.js instance rather than modifying the original. Assign the result to a new variable or chain methods.
Fix: Store the result of manipulation: `const newDay = originalDay.add(1, 'day');`
gotcha The `diff()` method returns an integer difference by default. To get a floating-point number for the difference, you must pass `true` as the third argument.
Fix: Use `dayjsA.diff(dayjsB, 'month', true)` for fractional differences.
gotcha Day.js parsing is not as strict as some other libraries. For explicit and strict parsing of specific date formats, the `CustomParseFormat` plugin is often required.
Fix: Import and use the `CustomParseFormat` plugin for precise parsing: `import customParseFormat from 'dayjs/plugin/customParseFormat'; dayjs.extend(customParseFormat); dayjs('12-25-1995', 'MM-DD-YYYY');`

Install

npm install dayjs npm
yarn add dayjs yarn
pnpm add dayjs pnpm

Imports

dayjs
wrong:
```
const dayjs = require('dayjs')
```
correct:
```
import dayjs from 'dayjs'
```
For ESM environments, use default import. CJS also supported.

Quickstart

Initializes a Day.js instance for the current date, then manipulates it by setting the year, adding a day, moving to the start of the month, and finally formats it into a specific string.

import dayjs from 'dayjs';

const formattedDate = dayjs().startOf('month').add(1, 'day').set('year', 2018).format('YYYY-MM-DD HH:mm:ss');

console.log(formattedDate);
// Example output: '2018-01-02 00:00:00'

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