1.0.4 • Public • Published



timezones without bloat

use moment-timezone? save 400KB, minified!

quick start

npm i tzjs
const d1 = new Date('2020-01-01') // Jan 1, midnight UTC
tzjs.getOffset('Europe/Berlin', d1) // -60
const d2 = new Date('2020-06-01') //  June 1, now we're on daylight savings
tzjs.getOffset('Europe/Berlin', d2) // -120
tzjs.getOffset('America/Los_Angeles', new Date()) // current offset in LA/SF
// Returns 420 or 480, depending on whether we're on daylight savings
const o1 = {hour: 'numeric', minute: '2-digit', timeZoneName: 'short'}
// example: "4:00 PM PST". localizes to the user's timezone and language
const o2 = {hour: 'numeric', minute: '2-digit', timeZone: 'UTC'}
tzjs.fmt(o2, 'en-US').format(d1)
// always returns "12:00 AM", regardless of browser settings
const o3 = {month: 'long', day: 'numeric', timeZone: 'UTC'}
tzjs.fmt(o3, 'es').format(d1)
// always returns "1 de enero"

support all the world's timezones, including DST and historical changes, in under 1KB of javascript!

getOffset is a few lines of very tricky code, working around a limitation in window.Intl.

fmt is a thin wrapper around Intl.DateTimeFormat. it's shorter to type and memoized, which we've measured to be important for performance.


the two most popular libraries for dealing with dates in the browser are moment and date-fns.

if you needed to format times in a specific timezone--any timezone other than browser local or UTC--the situation was grim. until now!

formatting library min. size timezone support lib min. size
moment 227KB moment-timezone 407KB
date-fns 29KB doesn't exist yet -
window.Intl.DateTimeFormat 0KB tzjs 1KB

you can use tzjs by itself or together with date-fns

api reference

tzjs exports just three functions.

import { getOffset, fmt, toDate } from 'tzjs'

or, with ES5,

const tzjs = require('tzjs')

getOffset(timeZone, date)

returns offset from UTC in minutes

note that if a timzone's at UTC+1, getOffset() returns -60, not 60

in other words, it returns the number of minutes you'd have to add to get to UTC

this behavior matches Date.getTimezoneOffset


getOffset('America/Los_Angeles', '2020-04-20')
// returns 420

fmt(options, locale)

returns Intl.DateTimeFormat(locale, options)

memoizes the result. this is important, since the DateTimeFormat constructor is slow.

locale is optional and defaults to the browser locale.


const opts = {
  year: 'numeric',
  month: 'short',
  day: '2-digit',
  hour: 'numeric',
  minute: 'numeric'
const optsWithTz = Object.assign({timeZone: 'America/New_York'}, opts)
fmt(optsWithTz, 'en-US').format(new Date('2020-01-01T00:00:00Z'))
// always returns "Dec 31, 2019, 7:00 PM"
fmt(opts, 'en-US').format(new Date('2020-01-01T00:00:00Z'))
// returns "Dec 31, 2019, 4:00 PM" here in California
// return value depends on browser timezone
fmt(optsTz, 'es').format(new Date('2020-01-01T00:00:00Z'))
// always returns "31 dic. 2019 19:00"
fmt(opts).format(new Date('2020-01-01T00:00:00Z'))
// might produce any of the values above!
// return value depends on browser timezone and language setting


helper function. takes a Date object, Unix millis, or an ISO timestamp like "2020-01-01T00:00:00Z". returns a Date.


fmt({year: 'numeric'}).format(toDate('2020-06-01'))
// returns "2020"


does it work in all browsers?

everywhere except Opera Mini and UC Browser for Android, mostly limited to China.

it even works in IE 11!

why is moment-timezone so big?

it's bundling much of the timezone database. this is big, complex and redundant: the browser already knows all of that information.

so why not just use browser APIs?

the Internationalization API has an omission so big that i think it qualifies as a bug.

you can print any instant in any timezone:

const options = {
  hour: '2-digit',
  timeZoneName: 'short',
  timeZone: 'Europe/Berlin'
const format = window.Intl.DateTimeFormat('en-US', options)
format.format(new Date('2020-01-01')) // Midnight Jan 1 UTC
// "1 AM GMT+1"
format.format(new Date('2020-06-01')) // Midnight June 1 UTC
// "2 AM GMT+2" (because of DST)

so under the hood, browser must already have the timezone database, and must already be calculating offsets. however, the offsets are not exposed in the API.

then how does tzjs work?

we work around this omission via a cute hack. we use window.Intl to format a given instant in both UTC and the target timezone. then, we parse both formatted times and subtract. turns out you can do this in just a few lines of code. see index.js.

is it fast?

reasonbly. all that string parsing is slower than moment but fast enough that it would be 0% of your render time in most UIs.

on my 2011-era Thinkpad x220:

$ npm run bench
Computing offsets for 744 dates x 493 zones
tzjs.getOffset: 366792 offsets in 5946ms
momentTz.utcOffset: 366792 offsets in 386ms

that works out to 16us per offset for tzjs vs 1us for moment-timezone

tzjs saves you ~400KB from your minified bundle size, which can cut page load time significantly.

date formatting

so the built-in Intl.DateTimeFormat is lots lighter than moment. however, writing DateTimeFormats everywhere can get cumbersome.

not only that, it can be slow. we've seen the DateTimeFormat constructor show up in profiles, taking up more than half of our frontend CPU!

solution: tzjs.fmt

we've found the following to work well. specifically, it's fast and light.

create a file that can be shared across your frontend. ours is called date.js:

/* @flow */
import { fmt, toDate } from 'tzjs'
 * Returns eg "Sun, Mar 11, 11:55pm PST"
export function dateTimeTz (d: string | Date, timeZone?: ?string): string {
  return fmt('en-US', {
    weekday: 'short',
    month: 'short',
    day: 'numeric',
    hour: 'numeric',
    minute: 'numeric',
    hour12: true,
    timeZoneName: 'short',
 * Returns eg "3/11" for March 11
export function dateMD(d: string | Date, timeZone?: ?string): string {
  return fmt('en-US', {
    month: 'numeric',
    day: 'numeric',

then, elsewhere:

/* @flow */
import { dateTimeTz } from '../date.js'
// ...
function MessageLabel (date: Date) {
  return <span>Sent {dateTimeTz(date, 'America/Los_Angeles')}</span>


npm i tzjs

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