Reduce moment-timezone data size for a webpack build
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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.


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).
    • It’s safe to keep only the data required for those zones (using the matchZones option).

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 three 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.

Version support

This plugin has been tested with and supports webpack v4. 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',

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,


MIT License © Gilmore Davidson