Dayex

A modern date kernel for product work — not a calendar widget, not a Moment clone.

Dayex is a tiny, immutable JavaScript date library. The core stays under 3 KB gzip. Everything else — locales, time zones, durations, business calendars, date ranges — loads only when you ask for it.

The name is deliberate: day + next. A date you can chain, a range you can hold, a working calendar you can actually ship.

Documentation · npm · API

import dayex from 'dayex'

dayex('2026-01-15')
  .startOf('month')
  .add(1, 'week')
  .format('YYYY-MM-DD')
// '2026-01-08'

Why Dayex

Classic date libraries treated time as a single mutable instant. Dayex treats time as values you compose.

| You need | Dayex gives you | | --- | --- | | A point in time | dayex() — parse, format, add, query | | A working calendar | businessDay — holidays, weekends, SLA offsets | | An interval | range — overlap, intersect, iterate, range sets | | A locale or zone | On-demand modules. Unused code never ships |

The parse / format / add surface will feel familiar if you have used Dayex. The model is not the same. Instances never mutate. Ranges are first-class objects. Business days are a real calendar, not day() !== 0.

Zero runtime dependencies. Time zones go through Intl, not a bundled tz database.

Install

npm install dayex

import dayex from 'dayex'
// or
const dayex = require('dayex')

Core

Every call returns a new instance.

const due = dayex('2026-03-01T09:00:00')

due.add(2, 'week').subtract(1, 'day')
due.startOf('month').endOf('week')
due.set('hour', 18).hour()          // 18
due.isBefore(dayex(), 'day')
due.format('YYYY-MM-DD HH:mm')

Parse a value, display it, move it, or compare it. Units accept long, short, and plural forms (day, d, days).

dayex()                             // now
dayex('2026-09-11')
dayex(Date.now())
dayex.unix(1773129600)

Locales

Nothing locale-specific is in the default build. Import what the product actually shows.

import 'dayex/locale/ko'
import 'dayex/locale/ja'

dayex.locale('ko')
dayex().format('YYYY년 MMMM D일 dddd')

dayex('2026-01-01').locale('ja').format('LL')

Plugins

Extend the kernel only for the features you use.

import dayex from 'dayex'
import utc from 'dayex/plugin/utc'
import timezone from 'dayex/plugin/timezone'
import duration from 'dayex/plugin/duration'
import relativeTime from 'dayex/plugin/relativeTime'

dayex.extend(utc)
dayex.extend(timezone)
dayex.extend(duration)
dayex.extend(relativeTime)

dayex('2026-01-01T00:00:00Z')
  .tz('Asia/Seoul')
  .from(dayex())

Other official modules include customParseFormat, isoWeek, isBetween, minMax, localizedFormat, advancedFormat, quarterOfYear, and calendar.

Two modules define the current Dayex surface — the parts a modern app actually argues about.

Business days

Weekends and holidays are not the same thing. businessDay keeps them separate, then lets you move and count on the result.

import businessDay from 'dayex/plugin/businessDay'

dayex.extend(businessDay, {
  workingWeekdays: [1, 2, 3, 4, 5],
  holidays: ['2026-01-01', { date: '12-25', name: 'Christmas' }]
})

dayex('2026-01-02T17:00:00').addBusinessDays(3)
// 2026-01-07T17:00:00  — weekend skipped, time kept

dayex('2026-01-03').toBusinessDay('nearest')
dayex('2026-01-31').businessDiff('2026-01-01')
dayex().businessDaysInMonth()

Override the calendar per call when one process serves more than one jurisdiction.

dayex('2026-07-03').isBusinessDay({ holidays: usFederal })

Ranges

Two timestamps are not an interval. range is a value: contains, overlaps, intersect, walk, split.

import range from 'dayex/plugin/range'

dayex.extend(range)

const leave = dayex.range('2026-01-05', '2026-01-09')
const sprint = dayex('2026-01').toRange('month')

leave.contains(dayex())
leave.overlaps(sprint)
leave.each('day').map((d) => d.format('MM/DD'))

// checkout day can be reused
const a = dayex.range('2026-01-01', '2026-01-03')
const b = dayex.range('2026-01-03', '2026-01-05')
a.overlaps(b, 'day', '[)')  // false
a.adjacent(b, 'day', '[)')  // true

Disconnected pieces become a RangeSet — merge, subtract, query as one calendar.

dayex.rangeSet(leave, dayex.range('2026-02-01', '2026-02-03'))

Design

• Immutable. add never changes the instance you already hold.
• Composable. Points, ranges, and working days are separate types that plug together.
• Paid for only once. Core has no dependencies. Plugins and locales are import-level.
• Host Intl for zones. No shipped timezone dump.
• Typed. Official plugins ship declaration files.

The chainable grammar is intentionally close to the ecosystem people already type. The types you get back — especially DayexRange and a configured business calendar — are the modern part.

Docs

Full parse, display, and plugin reference: dayex.org

License

MIT. See LICENSE.