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Oof, that’s a clunky name, but at least it’s descriptive.

This is a plugin for webpack which reduces data for moment-timezone.

Why is this needed?

Moment Timezone is a comprehensive library for working with time zones in JavaScript. But that comprehensiveness comes with a file size cost. The full time zone data file is 903KiB raw, or 36KiB minified and gzipped (as of moment-timezone version 0.5.23).

That’s a lot of data to send to someone’s browser, especially if you don’t need all of it. Some of the time zones have data dating back to the 19th century. Thankfully there is an API to produce a custom data bundle containing only the time zone definitions you require.

Unfortunately, if you’re building your project with webpack, you don’t get to use a custom data bundle. A webpack build uses the Node.js version of moment-timezone, which automatically includes all the time zone data. Even if you configure Moment Timezone to use a custom data bundle at run-time, the full data file will still be present in your JavaScript bundle.

This plugin allows you to configure which time zone data you want. Any unwanted data is then automatically stripped from the compiled JS bundle at build time.

Use it in combination with the moment-locales-webpack-plugin to further reduce the compiled JS bundle size.

Project status

As of late 2020, the Moment and Moment Timezone projects are in maintenance-only mode. But they are still getting occasional updates, especially for new time zone data. This plugin will be maintained as long as both webpack and Moment Timezone are maintained.


Take a super-simple file which does nothing more than require('moment-timezone'). Building this with webpack in production mode results in over 1 MiB of minified JS code.

What if you only need the default English locale, and time zone data for Australia and New Zealand from 2018 to 2028? (This is a realistic scenario from a recent project.)

Running webpack in production mode results in the following file sizes:

Configuration Raw size Gzipped
Default 1164 KiB 105 KiB
Strip locales 959 KiB (~82%) 56 KiB (~53%)
Strip tz data 265 KiB (~23%) 69 KiB (~66%)
Strip locales & tz data 60 KiB (~5%) 20 KiB (~19%)

(Testing done with webpack@4.28.3, moment@2.23.0, moment-timezone@0.5.23.)

Even if you still need all the time zones available, reducing the data to a much smaller date range can produce significant file size savings. Building the above example file with data for all zones from 2018 to 2028 produces a file size of 288KiB, or 74KiB gzipped.

⚠️ Make sure you know what you’re doing ❗️️

Dealing with time zones can be tricky, and bugs can pop up in unexpected places. That’s doubly true when you’re auto-removing data at build time. When using this plugin, make absolutely sure that you won’t need the data you’re removing.

For example, if you know for certain that your web site/application...

  1. ...will never deal with past dates & times earlier than a certain point (e.g. the launch date of the application).
  2. ...will never deal with future dates & times beyond a certain point (e.g. details of a specific event).
    • It’s (relatively) safe to remove any data beyond that date (using the endYear option).
  3. ...will only deal with a fixed set of time zones (e.g. rendering times relative to a set of physical buildings in a single country).

However, if you’re allowing users to choose their time zone preference — with no theoretical limit on the range of dates you’ll handle — then you’re going to need all the data you can get.

If you’re in doubt about whether to include some data, err on the side of caution and include it.



Using npm:

npm install --save-dev moment-timezone-data-webpack-plugin

Or using yarn:

yarn add --dev moment-timezone-data-webpack-plugin


Add the plugin to your webpack config file:

const MomentTimezoneDataPlugin = require('moment-timezone-data-webpack-plugin');

module.exports = {
  plugins: [
    new MomentTimezoneDataPlugin({
      // options

Plugin options

There are four available options to filter the time zone data. At least one option must be provided.

  • startYear (integer) — Only include data from this year onwards.
  • endYear (integer) — Only include data up to (and including) this year.
  • matchZones — Only include data for time zones with names matching this value. matchZones can be any of these types:
    • string — Include only this zone name as an exact match (e.g. 'Australia/Sydney').
    • regexp — Include zones with names matching the regular expression (e.g. /^Australia\//).
    • array (of the above types) — Include zones matching any of the values of the array. Each value can be a string or a regular expression, which will be matched following the rules above.
  • matchCountries — Only include data for time zones associated with specific countries, as determined by Moment Timezone’s zonesForCountry() API. matchCountries works with ISO 3166 2-letter country codes, and can be any of these types:
    • string — Include zones for this country code as an exact match (e.g. 'AU').
    • regexp — Include zones for country codes matching the regular expression (e.g. /^A|NZ/).
    • array (of the above types) — Include zones for country codes matching any of the values of the array. Each value can be a string or a regular expression, which will be matched following the rules above.

NOTE: The matchCountries option will only work when used with moment-timezone version 0.5.28 or later. If this option is used with a non-compliant version of moment-timezone, an error will be thrown.

All filtering options are AND (a.k.a. conjunction) filters — that is, they become more restrictive as each one is applied. Only zone data that match all the provided filters will be added to the final output. For this reason, it’s probably safer to provide only one of matchZones or matchCountries; providing both is allowed, but you may not get the results you expect.

There are also some non-filtering options that can be provided to configure other behaviour around file locations.

  • cacheDir (string) — A path where the generated files will be cached. If not provided, the files will be cached in an automatically-generated location.
  • momentTimezoneContext (regexp) — A regexp matching a context where moment-timezone is located. The timezone file will be replaced only if it is located in this context. Other instances of the timezone file out of this context will not be touched. This is useful in case you are using a version stored outside of node_modules (e.g. if module or vendor directory is vendor\moment-timezone the context could be matched for example with vendor[\\/]moment-timezone$). Defaults to /node_modules[\\/]moment-timezone$/.

Version support

This plugin has been tested with and officially supports the following dependencies:

  • Node.js 8 or higher
  • webpack 4 and 5
  • moment-timezone v0.1.0 or higher

It theoretically supports older versions of webpack (as it uses built-in webpack plugins internally), but this hasn’t been tested.


All zones, with a limited date range

const currentYear = new Date().getFullYear();
const plugin = new MomentTimezoneDataPlugin({
  startYear: currentYear - 2,
  endYear: currentYear + 10,

All data for a specific zone

const plugin = new MomentTimezoneDataPlugin({
  matchZones: 'America/New_York',

All data for a specific country

const plugin = new MomentTimezoneDataPlugin({
  // Includes 'Pacific/Auckland' and 'Pacific/Chatham'
  matchCountries: 'NZ',

Limited data for a set of zones (single regular expression)

const plugin = new MomentTimezoneDataPlugin({
  matchZones: /Europe\/(Belfast|London|Paris|Athens)/,
  startYear: 2000,
  endYear: 2030,

Limited data for a set of zones (array of values)

const plugin = new MomentTimezoneDataPlugin({
  matchZones: [/^Australia/, 'Pacific/Auckland', 'Etc/UTC'],
  startYear: 2000,
  endYear: 2030,

Limited data for a set of countries

const plugin = new MomentTimezoneDataPlugin({
  matchCountries: ['US', 'CA'],
  startYear: 2000,
  endYear: 2030,

How zone links are handled

Moment Timezone has the concept of zone links, which are simple aliases from one zone name to another. These roughly match the Zone and Link definitions in the underlying IANA time zone database files. (But they don't exactly match the tzdb links, for complicated reasons.)

This plugin will automatically include linked zones in some circumstances:

  1. The matchZones option will include only zones that match the provided value(s). If a zone is defined in Moment Timezone as a link, then the zone it points to is also included.
  2. The matchCountries option chooses which zones are included based on the countries data provided by Moment Timezone.
    1. Those entries originally come from the and files in the IANA time zone database.
    2. As with matchZones, there is some link handling here, but only if the links are found in that primary countries list.
  3. If an included zone has other links pointing to it, those links won't be included. This was done because some zones have many, many links (e.g. America/Puerto_Rico has 20 links pointing to it). This has become more prevalent in recent years as the tzdb has been merging many zones together across country boundaries.

For example, the zone US/Eastern is a backwards-compatibility link to America/New_York. Because it's a deprecated name, US/Eastern doesn't appear in the zone*.tab files, but America/New_York does. Therefore:

  • Using matchZones: 'US/Eastern' will include US/Eastern and America/New_York.
  • Using matchZones: 'America/New_York will include only America/New_York.
  • Using matchCountries: 'US' will include America/New_York, America/Detroit, America/Los_Angeles, etc., but will not include US/Eastern.


MIT License © Gilmore Davidson