timezone-date.ts

Enhancement of Date class with better timezone support.

Features

• Compliant to Date methods
• Allow changing timezone anytime
• Support both numeric timezone offsets and IANA timezone identifiers (e.g., "America/New_York")
• Automatic Daylight Saving Time (DST) handling with IANA timezones
• Typescript support
• Tiny code base (below 1KB minizipped)

Example

Jump between timezones:

import { TimezoneDate } from 'timezone-date.ts'

const d = TimezoneDate.fromDate(new Date('2020-04-21T10:00:00.000Z'))

d.timezone = 0
d.getHours() // 10

d.timezone = +8
d.getHours() // 18

d.getTimezoneOffset() // -480 (in mintes, same format as Native Date)

d.setHours(9)
d.timezone++
d.getHours() // 10
d.toLocaleTimeString() // '10:00:00 AM'

Set a specific time (e.g. from UI input)

let date = new TimezoneDate()
date.timezone = +8
date.setFullYear(2020, 11 - 1, 28)
date.setHours(9, 2, 38)
console.log(date.toString()) // Sat Nov 28 2020 09:02:38 GMT+0800 (Hong Kong Standard Time)

Using IANA timezone identifiers with automatic DST handling:

import { TimezoneDate } from 'timezone-date.ts'

let date = new TimezoneDate()
date.timezone = 'America/New_York' // IANA timezone identifier

// Summer time (EDT, UTC-4)
date.setTime(new Date('2025-06-01T12:00:00Z').getTime())
date.getTimezoneOffset() / 60 // -4 (EDT)
date.toLocaleString('en-US') // "6/1/2025, 8:00:00 AM"

// Winter time (EST, UTC-5)
date.setTime(new Date('2025-12-01T12:00:00Z').getTime())
date.getTimezoneOffset() / 60 // -5 (EST)
date.toLocaleString('en-US') // "12/1/2025, 7:00:00 AM"

Using the exported utility function:

import { getTimezoneOffsetForIANA } from 'timezone-date.ts'

const time = new Date('2025-06-01T12:00:00Z').getTime()
const offset = getTimezoneOffsetForIANA('America/New_York', time)
console.log(offset) // -4 (hours, EDT in summer)

Installation

npm i timezone-date.ts

Construction

import { TimezoneDate } from 'timezone-date.ts'

/* custom timezone (numeric offset) */
new TimezoneDate(Date.now(), { timezone: +8 })

/* custom timezone (IANA identifier) */
new TimezoneDate(Date.now(), { timezone: 'America/New_York' })

/* default timezone (in the environment) */
new TimezoneDate() // current time
new TimezoneDate(Date.now())
TimezoneDate.fromTime(Date.now())
TimezoneDate.fromDate(new Date())
TimezoneDate.from(Date.now()) // from Date or timestamp

Timezone Format

The timezone property accepts:

• Numeric offset (in hours): +8, -5, 0, etc.
  • Fixed offset, does not handle DST automatically
• IANA timezone identifier: "America/New_York", "Asia/Hong_Kong", "Europe/London", etc.
  • Automatically handles Daylight Saving Time (DST)
  • Uses the browser's/Node.js's built-in timezone database

Exported Functions

getTimezoneOffsetForIANA(timezone: string, time: number): number

Get the timezone offset in hours for a given IANA timezone identifier at a specific time.

import { getTimezoneOffsetForIANA } from 'timezone-date.ts'

const time = new Date('2025-06-01T12:00:00Z').getTime()
const offset = getTimezoneOffsetForIANA('America/New_York', time)
// Returns -4 (EDT in summer) or -5 (EST in winter)

License

This is free open sourced software (FOSS), with BSD 2-Clause License