Moment.js / Day.js JavaScript (Node.js): Methods…

What is this library and why is it needed

Moment.js and Day.js are two powerful JavaScript libraries designed for working with dates and times. They solve a fundamental problem: the native Date object in JavaScript is extremely inconvenient for everyday tasks. It does not support formatting, complex date calculations, parsing strings in different formats, or working with time zones without writing dozens of lines of code.

Moment.js was the de facto standard for a long time, but is now considered "legacy" — its development is frozen, and it is not recommended for new projects due to its large size (about 300 KB). It has been succeeded by Day.js — a lightweight (only 2 KB) and fast library with a nearly identical API. Day.js is fully compatible with Moment.js for most basic operations, making migration easy.

Both libraries provide:

Parsing dates from strings in any format
Formatting dates into human-readable strings
Manipulations (adding/subtracting days, months, years)
Date comparison (before, after, between)
Time zone support (via plugins)
Localization (support for different languages and regional formats)

In this article, we will focus on Day.js as a modern alternative, but all examples will work with Moment.js with minimal changes.

Installation

Installation for Node.js is done via npm or yarn. For the browser, you can use a CDN.

Installing Day.js

# Via npmnpm install dayjs

# Via yarnyarn add dayjs

Installing Moment.js

npm install moment

# oryarn add moment

Importing in a project

// Day.jsconst dayjs = require('dayjs');

// Moment.jsconst moment = require('moment');

Quick start with code example

After installation, you can start working with dates immediately. Here is a minimal example demonstrating the main features:

const dayjs = require('dayjs');

// Current date and timeconst now = dayjs();console.log('Now:', now.format('DD.MM.YYYY HH:mm:ss'));

// Parsing a stringconst date = dayjs('2024-03-15');console.log('Parsing:', date.format('DD.MM.YYYY'));

// Manipulationsconst nextWeek = dayjs().add(7, 'day');console.log('Next week:', nextWeek.format('DD.MM.YYYY'));

// Difference between datesconst diff = nextWeek.diff(now, 'day');console.log('Difference in days:', diff);

// Comparisonconsole.log('Is today earlier?', now.isBefore(nextWeek)); // true

Main methods/functions

1. dayjs() — creating a date object

What it does: Creates a Day.js object representing a specific point in time. Without arguments, it returns the current date and time.

Parameters:

date (optional) — a string, number (timestamp), Date object, or another Day.js object
format (optional) — a format string for parsing, if date is a string

Working code example:

// Current dateconst now = dayjs();

// From a stringconst fromString = dayjs('2024-12-31');

// From a timestamp (milliseconds)const fromTimestamp = dayjs(1704067200000);

// From a Date objectconst fromDate = dayjs(new Date(2024, 0, 1)); // January 1, 2024

// With format specificationconst customFormat = dayjs('31.12.2024', 'DD.MM.YYYY');

console.log(now.format()); // 2024-03-15T10:30:00+03:00console.log(customFormat.format('YYYY-MM-DD')); // 2024-12-31