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date-fns-tz
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date-fns-tz

Complementary library for date-fns v2 adding IANA time zone support

by Marnus Weststrate

1.3.5 (see all)License:MITTypeScript:Built-InCategories:Vanilla JavaScript Timezone
npm i date-fns-tz
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date-fns-tz

Time zone support for date-fns v2.0.0 using the Intl API. By using the browser API no time zone data needs to be included in code bundles. Modern browsers and Node.js all support the necessary features, and for those that don't a polyfill can be used.

If you do not wish to use a polyfill the time zones can still be specified as offsets such as '-0200' or '+04:00', but not IANA time zone names.

Note: date-fns is a peer dependency of this library.

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ESM and CommonJS

As of v1.3.0 this library supports native ESM imports in Node.js. It is important to use the proper import path for your use case. Set the type property in your project's package.json to either module, for ESM, or commonjs and

  • For ESM import from date-fns-tz/esm or its sub-paths
  • For CommonJS import from date-fns-tz or its sub-paths

Even when using ESM some CommonJS imports from date-fns will be used until they support ESM natively as well date-fns#1781. This is because an ESM projects cannot use ESM imports from a library that doesn't specify {"type": "module"}.

Table of Contents

Overview

Working with UTC or ISO date strings is easy, and so is working with JS dates when all times are displayed in a user's local time in the browser. The difficulty comes when working with another time zone's local time, one other than the current system's, like on a Node server or when showing the time of an event in a specific time zone, like an event in LA at 8pm PST regardless of where a user resides.

In this case there are two relevant pieces of information:

  • a fixed moment in time in the form of a timestamp, UTC or ISO date string, and
  • the time zone descriptor, usually an offset or IANA time zone name (e.g. America/New_York).

Libraries like Moment and Luxon, which provide their own date-time classes, manage these timestamp and time zone values internally. Since date-fns always returns a plain JS Date, which implicitly has the current system's time zone, helper functions are provided for handling common time zone related use cases.

Date and time zone formatting

formatInTimeZone

This function takes a Date instance in the system's local time or an ISO8601 string, and an IANA time zone name or offset string. It then formats this date in the target time zone regardless of the system's local time zone.

It supports the same format tokens as date-fns/format, and adds full support for:

  • The z..zzz Unicode tokens: short specific non-location format, e.g. EST
  • The zzzz Unicode token: long specific non-location format, e.g. Eastern Standard Time

Unlike date-fns/format, the z..zzzz, x..xxxxx, X..XXXXX and O..OOO tokens will all print the formatted value of the provided time zone rather than the system time zone.

An invalid date or time zone input will result in an Invalid Date passed to date-fns/format, which will throw a RangeError.

For most use cases this is the only function from this library you will need.

import { formatInTimeZone } from 'date-fns-tz'

const date = new Date('2014-10-25T10:46:20Z')

formatInTimeZone(date, 'America/New_York', 'yyyy-MM-dd HH:mm:ssXXX') // 2014-10-25 06:46:20-04:00
formatInTimeZone(date, 'America/New_York', 'yyyy-MM-dd HH:mm:ss zzz') // 2014-10-25 06:46:20 EST
formatInTimeZone(date, 'Europe/Paris', 'yyyy-MM-dd HH:mm:ss zzz') // 2014-10-25 10:46:20 GMT+2

// The time zone name is generated by the Intl API which works best when a locale is also provided
import enGB from 'date-fns/locale/en-GB'

formatInTimeZone(parisDate, 'Europe/Paris', 'yyyy-MM-dd HH:mm:ss zzz', { locale: enGB })
// 2014-10-25 10:46:20 CEST

formatInTimeZone(parisDate, 'Europe/Paris', 'yyyy-MM-dd HH:mm:ss zzzz', { locale: enGB })
// 2014-10-25 10:46:20 Central European Summer Time

Time zone offset helpers

These functions are useful when you are not formatting a date yourself, but passing it to third-party code such as a date picker library alongside an input for selecting a time zone.

To discuss the usage of the time zone helpers let's assume we're writing a system where administrators set up events which will start at a specific time in the venue's local time, and this local time should be shown when accessing the site from anywhere in the world.

zonedTimeToUtc

Given a date and any time zone, returns a Date with the equivalent UTC time. An invalid date string or time zone will result in an Invalid Date.

zonedTimeToUtc(date: Date|Number|String, timeZone: String): Date

Say a user is asked to input the date/time and time zone of an event. A date/time picker will typically return a Date instance with the chosen date, in the user's local time zone, and a select input might provide the actual IANA time zone name.

In order to work with this info effectively it is necessary to find the equivalent UTC time:

import { zonedTimeToUtc } from 'date-fns-tz'

const date = getDatePickerValue() // e.g. 2014-06-25 10:00:00 (picked in any time zone)
const timeZone = getTimeZoneValue() // e.g. America/Los_Angeles

const utcDate = zonedTimeToUtc(date, timeZone) // In June 10am in Los Angeles is 5pm UTC

postToServer(utcDate.toISOString(), timeZone) // post 2014-06-25T17:00:00.000Z, America/Los_Angeles

utcToZonedTime

Returns a Date which will format as the local time of any time zone from a specific UTC time. An invalid date string or time zone will result in an Invalid Date.

utcToZonedTime(date: Date|Number|String, timeZone: String): Date

Say the server provided a UTC date/time and a time zone which should be used as initial values for the above form. The date/time picker will take a Date input which will be in the user's local time zone, but the date value must be that of the target time zone.

import { utcToZonedTime } from 'date-fns-tz'

const { isoDate, timeZone } = fetchInitialValues() // 2014-06-25T10:00:00.000Z, America/New_York

const date = utcToZonedTime(isoDate, timeZone) // In June 10am UTC is 6am in New York (-04:00)

renderDatePicker(date) // 2014-06-25 06:00:00 (in the system time zone)
renderTimeZoneSelect(timeZone) // America/New_York

getTimezoneOffset

Returns the offset in milliseconds between the time zone and UTC time.

getTimezoneOffset(timeZone: String, date: Date|Number): number

Returns the time zone offset from UTC time in milliseconds for IANA time zones as well as other time zone offset string formats.

For time zones where daylight savings time is applicable a Date should be passed on the second parameter to ensure the offset correctly accounts for DST at that time of year. When omitted, the current date is used.

For invalid time zones, NaN is returned.

import { getTimezoneOffset } from 'date-fns-tz'

const result = getTimezoneOffset('-07:00')
//=> -18000000 (-7 * 60 * 60 * 1000)
const result = getTimezoneOffset('Africa/Johannesburg')
//=> 7200000 (2 * 60 * 60 * 1000)
const result = getTimezoneOffset('America/New_York', new Date(2016, 0, 1))
//=> -18000000 (-5 * 60 * 60 * 1000)
const result = getTimezoneOffset('America/New_York', new Date(2016, 6, 1))
//=> -14400000 (-4 * 60 * 60 * 1000)

Low-level formatting helpers

format

The format function exported from this library is used under the hood by formatInTimeZone and extends date-fns/format with full time zone support for:

  • The z..zzz Unicode tokens: short specific non-location format
  • The zzzz Unicode token: long specific non-location format

When using those tokens with date-fns/format it falls back to the GMT time zone format, and always uses the current system's local time zone. For example zzz in New York will always return GMT-4 instead of the desired EST, and zzz in Paris GMT+2 instead of CEST, making the time zone tokens somewhat irrelevant. This extended format function returns the proper specific non-location format, e.g. EST or Eastern Standard Time, and that of the target time zone (if provided, see below) rather than the system time zone.

Since a JavaScript Date instance cannot convey the time zone information to the format function it is necessary to pass the timeZone value as an option on the third argument of format.

Similar to date-fns/format, when an invalid date is used a RangeError is thrown. When an invalid time zone is provided and included in the output, i.e. with time zone tokens in the format string, it will also throw a RangeError.

To format a date showing time for a specific time zone other than the system time zone, the format function can be combined with utcToZonedTime. This is what formatInTimeZone does internally. To clarify, the format function will never change the underlying date, it must be changed to a zoned time before passing it to format.

In most cases there is no need to use format rather than formatInTimeZone. The only time this makes sense is when utcToZonedTime has been applied to a date once, and you want to format it multiple times to different outputs.

import { format, utcToZonedTime } from 'date-fns-tz'

const date = new Date('2014-10-25T10:46:20Z')

const nyDate = utcToZonedTime(date, 'America/New_York')
const parisDate = utcToZonedTime(date, 'Europe/Paris')

format(nyDate, 'yyyy-MM-dd HH:mm:ssXXX', { timeZone: 'America/New_York' }) // 2014-10-25 06:46:20-04:00
format(nyDate, 'yyyy-MM-dd HH:mm:ss zzz', { timeZone: 'America/New_York' }) // 2014-10-25 06:46:20 EST
format(parisDate, 'yyyy-MM-dd HH:mm:ss zzz', { timeZone: 'Europe/Paris' }) // 2014-10-25 10:46:20 GMT+2

// The time zone name is generated by the Intl API which works best when a locale is also provided
import enGB from 'date-fns/locale/en-GB'

format(parisDate, 'yyyy-MM-dd HH:mm:ss zzz', {
  timeZone: 'Europe/Paris',
  locale: enGB,
})
// 2014-10-25 10:46:20 CEST
format(parisDate, 'yyyy-MM-dd HH:mm:ss zzzz', {
  timeZone: 'Europe/Paris',
  locale: enGB,
})
// 2014-10-25 10:46:20 Central European Summer Time

toDate

The toDate function can be used to parse a Date from a string containing a date and time representing time in any time zone by providing an IANA time zone name on the timeZone option.

An invalid date string or time zone will result in an Invalid Date.

import { toDate, format } from 'date-fns-tz'

// Offsets in the date string work as usual and take precedence
const parsedDate = toDate('2014-10-25T13:46:20+04:00')
const parisDate = utcToZonedTime(parsedDate, 'Europe/Paris')
format(parisDate, 'yyyy-MM-dd HH:mm:ssxxx', { timeZone: 'Europe/Paris' }) // 2014-10-25 11:46:20+02:00

// Since toDate simply clones a Date instance, the timeZone option is effectively ignored in this case
const date = new Date('2014-10-25T13:46:20Z')
const clonedDate = toDate(date, { timeZone: 'Europe/Paris' })
assert(date.valueOf() === clonedDate.valueOf())

// When there is no offset in the date string the timeZone property is used
const parsedDate = toDate('2014-10-25T13:46:20', { timeZone: 'Asia/Bangkok' })
const bangkokDate = utcToZonedTime(parsedDate, 'Asia/Bangkok')
format(bangkokDate, 'yyyy-MM-dd HH:mm:ssxxx', { timeZone: 'Asia/Bangkok' }) // 2014-10-25 13:46:20+07:00

Usage with Android

This library works with React Native, however the Intl API is not available by default on Android.

In projects that do not use Hermes, make this change to android/app/build.gradle:

- def jscFlavor = 'org.webkit:android-jsc:+'
+ def jscFlavor = 'org.webkit:android-jsc-intl:+'

React Native does not currently support Intl on Android with Hermes (facebook/hermes#23). The best bet seems to be using the polyfills by Format.JS.

Usage with Node.js

Node.js supports the Intl API and ships with full ICU data included in the binary from v13, i.e. this library will just work.

Node.js v12, which reaches end of life on 30 April 2022, requires running with full ICU data provided at runtime.

Credit

The idea of using the Intl API for time zone support was inspired by the Luxon library.

The initial port of the idea into date-fns was done by @benmccan in date-fns/#676.

License

MIT © Marnus Weststrate