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

date-fns-tz

Time zone support for date-fns v2.0.0.

Dependency free IANA time zone support is implemented via the Intl API to keep actual time zone data out of code bundles. Modern browsers 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 zone option can still be used, but only with time zone offsets such as '-0200' or '+04:00' and not IANA time zone names.

This work was initially proposed in PR date-fns/#707, but won't be considered until date-fns version 2 has been released. It is my hope that these features will eventually make it into date-fns or at least contribute to the conversation and that this project will be deprecated.

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, 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. Sine 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.

Time Zone Helpers

To discuss the usage of the time zone helpers let's assume we're writing a system where administrators set up events which will be 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

Get a date with the correct UTC time for the date/time in a specific time zone

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

Get a date/time in the local time of any time zone from UTC time

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

Time Zone Formatting

format

The format function exported from this library extends date-fns/format with full time zone support:

  • The z..zzz Unicode tokens: short specific non-location format
  • The zzzz Unicode token: long specific non-location format
  • Using the name of any IANA time zone or offset by specifying a timeZone option; when using this option the x..xxxxx, X..XXXXX and O..OOO tokens will also use the provided time zone rather than the system time zone.
import { format, utcToZonedTime } from 'date-fns-tz'

const date = new Date('2014-10-25T10:46:20Z')
const nyTimeZone = 'America/New_York'
const parisTimeZone = 'Europe/Paris'

const nyDate = utcToZonedTime(date, nyTimeZone)
const parisDate = utcToZonedTime(date, parisTimeZone)

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'
enGB.code = '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

Caveat: Note that when using a locale the language code of the locale should be added to the import somewhere in the project so format can identify the locale. Once this library is absorbed into date-fns this can be added to each locale natively.

toDate

The toDate function can be used to create a zoned Date from a string containing an offset or IANA time zone, or by providing the timeZone option.

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

// Offsets in the date string work as usual and take precedence
const parisDate = toDate('2014-10-25T13:46:20+02:00')
format(parisDate, 'yyyy-MM-dd HH:mm:ssZ', { timeZone: 'Europe/Paris' }) // 2014-10-25 13:46:20+02:00

// Since toDate simply clones a Date instance 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 bangkokDate = toDate('2014-10-25T13:46:20', { timeZone: 'Asia/Bangkok' })
format(bangkokDate, 'yyyy-MM-dd HH:mm:ssZ', { timeZone: 'Asia/Bangkok' }) // 2014-10-25 13:46:20+07:00

const nyDate = toDate('2014-10-25T13:46:20 America/New_York')
format(nyDate, 'yyyy-MM-dd HH:mm:ssZ', { timeZone: 'America/New_York' }) // 2014-10-25 13:46:20-04:00

Note: Since the Intl API does not provide a way to parse long or short time zone names the parse function cannot be supported using this approach.

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.

Alternatives

The date-fns-timezone library provides similar functionality for date-fns version 1 by bundling time zone data. This does have the advantage of making parsing time zone long and short names possible.

License

MIT © Marnus Weststrate

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