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Day.js

A 2 KB Moment.js replacement with a nearly identical API.

Developer UtilitiesUtilityJavaScript
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What it is

Day.js is a minimalist JavaScript library for parsing, validating, manipulating, and formatting dates. It provides a modern API similar to Moment.js but with a much smaller footprint.

Day.js offers simple functions for working with dates, including parsing, formatting, adding or subtracting time, comparing dates, and working with relative times or durations.

Licence
MIT
Best known for
Moment's API in a fraction of the size

When to use it

The question documentation cannot answer for you — because it cannot recommend something else.

Reach for it when

  • Migrating away from Moment with minimal code changes
  • You want chainable date formatting and bundle size matters

Look elsewhere when

  • You need extensive timezone or locale work without pulling in several plugins

Installation

npm install dayjs

Getting started

The smallest useful thing you can do with it, and what each part means.

Getting current date and formatting
import dayjs from 'dayjs';
const now = dayjs();
console.log(now.format('YYYY-MM-DD HH:mm:ss'));
Gets the current date and formats it as a string.
Parsing a date string
const date = dayjs('2025-08-22');
console.log(date.format('MMMM D, YYYY'));
Parses a string into a Day.js object and formats it.

Advanced usage

Where the library earns its place over a simpler alternative.

Adding and subtracting time
const future = dayjs().add(7, 'day').subtract(2, 'hour');
console.log(future.format());
Adds 7 days and subtracts 2 hours from the current date.
Comparing dates
const d1 = dayjs('2025-08-22');
const d2 = dayjs('2025-09-01');
console.log(d1.isBefore(d2)); // true
console.log(d1.isAfter(d2)); // false
Compares two dates to check if one is before or after the other.
Relative time
import relativeTime from 'dayjs/plugin/relativeTime';
dayjs.extend(relativeTime);
const past = dayjs('2025-08-15');
console.log(past.fromNow());
Displays relative time from now, e.g., '7 days ago', using the relativeTime plugin.
Working with UTC
import utc from 'dayjs/plugin/utc';
dayjs.extend(utc);
const utcTime = dayjs.utc('2025-08-22 12:00');
console.log(utcTime.format());
Uses the UTC plugin to handle dates in Coordinated Universal Time.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Invalid Date
Ensure the input string is in a valid format or use `dayjs(string, format)` with the correct format specified.
Plugin not loaded
Extend Day.js with required plugins before using their functions, e.g., `dayjs.extend(plugin)`.

Best practices

  • Use plugins only for features you need to minimize bundle size.
  • Prefer Day.js for modern JavaScript projects requiring small date libraries.
  • Use immutable objects to avoid unintended mutations.
  • Use ISO 8601 strings when possible for consistent parsing.
  • Combine with timezone plugins for handling different time zones.

Alternatives

Comparable options, and the reason you would pick one over the other.

Background

Why it exists, and what it was reacting to.

Day.js was created to provide a lightweight alternative to Moment.js while maintaining a familiar API. Its modular architecture allows importing only the features you need, reducing bundle size significantly for web applications.

Related libraries

Others in Developer Utilities for JavaScript.

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