64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
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# Time Zones
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## Table of Contents
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- [Overview](#overview)
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- [`date-fns-tz`](#date-fns-tz)
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## Overview
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Working with UTC or ISO date strings is easy, and so is working with JS dates when all times
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are displayed in a user's local time in the browser. The difficulty comes when working with another
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time zone's local time, other than the current system's, like showing the local time of an event in LA
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at 8pm PST on a Node server in Europe or a user's machine set to EST.
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In this case there are two relevant pieces of information:
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- a fixed moment in time in the form of a timestamp, UTC or ISO date string, and
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- the time zone descriptor, usually an offset or IANA time zone name (e.g. `America/Los_Angeles`).
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Libraries like Moment and Luxon, which provide their own date time classes, manage these timestamp and time
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zone values internally. Since `date-fns` always returns a plain JS Date, which implicitly has the current
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system's time zone, helper functions are needed for handling common time zone related use cases.
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## [`date-fns-tz`](https://www.npmjs.com/package/date-fns-tz)
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Dependency free IANA time zone support is implemented via the
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[Intl API](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl) to keep
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actual time zone data out of code bundles. Modern browsers all support the
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[necessary features](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat#Browser_compatibility),
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and for those that don't a [polyfill](https://github.com/yahoo/date-time-format-timezone) can be used.
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Functions are provided for converting to and from a Date instance which will have the internal UTC time
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adjusted so it prints to the correct time value in the associated time zone, regardless of the current
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system time zone. The `date-fns` `format` function is extended with support for the `z...zzzz` tokens to
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format long and short time zone names.
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Compatible with `date-fns` version 2
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License: MIT
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### Synopsis
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```js
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const { zonedTimeToUtc, utcToZonedTime, format } = require('date-fns-tz')
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// Set the date to "2018-09-01T16:01:36.386Z"
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const utcDate = zonedTimeToUtc('2018-09-01 18:01:36.386', 'Europe/Berlin')
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// Obtain a Date instance that will render the equivalent Berlin time for the UTC date
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const date = new Date('2018-09-01T16:01:36.386Z')
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const timeZone = 'Europe/Berlin'
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const zonedDate = utcToZonedTime(date, timeZone)
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// zonedDate could be used to initialize a date picker or display the formatted local date/time
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// Set the output to "1.9.2018 18:01:36.386 GMT+02:00 (CEST)"
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const pattern = 'd.M.yyyy HH:mm:ss.SSS \'GMT\' XXX (z)'
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const output = format(zonedDate, pattern, { timeZone: 'Europe/Berlin' })
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```
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### Links
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- [API / Usage Scenarios](https://github.com/marnusw/date-fns-tz#time-zone-helpers)
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