$devvkit learn --librarie day.js-guide

Day.js Guide

[date][time][utilities]

JavaScript / TypeScript

Install

npm install dayjs

Day.js offers Moment.js-compatible API at a fraction of the size (2kB).

Plugin architecture keeps core small: add only what you need.

Immutable: calling .add() returns a new instance, avoiding mutation bugs.

Parsing & Display

Parse· Create from string.

import dayjs from 'dayjs';
dayjs();
dayjs('2026-07-13');
dayjs('2026-07-13', 'YYYY-MM-DD');

Format· Output formatted string.

dayjs().format('YYYY-MM-DD');     // '2026-07-13'
dayjs().format('MMM D, YYYY');     // 'Jul 13, 2026'

Add / subtract· Modify date.

dayjs().add(7, 'day');
dayjs().subtract(1, 'month');

Start of· Start of time unit.

dayjs().startOf('month'); // 2026-07-01 00:00:00
dayjs().endOf('month');

Difference· Calculate difference.

dayjs('2026-07-13').diff(dayjs('2026-01-01'), 'day'); // 193

Relative time· "3 hours ago".

import relativeTime from 'dayjs/plugin/relativeTime';
dayjs.extend(relativeTime);
dayjs('2026-07-10').fromNow(); // '3 days ago'

UTC· UTC mode.

import utc from 'dayjs/plugin/utc';
dayjs.extend(utc);
dayjs.utc().format();

Timezone· Convert timezones.

import timezone from 'dayjs/plugin/timezone';
dayjs.extend(utc);
dayjs.extend(timezone);
dayjs.tz('2026-07-13 12:00', 'America/New_York');

Locale· Display in language.

import 'dayjs/locale/fr';
dayjs.locale('fr');
dayjs('2026-07-13').format('dddd D MMMM');