Handling Time Zone in JavaScript

What is Time zone?

A time zone is a region that follows a uniform local time which is legally stated by the country. It’s common for many countries to have its unique time zone, and some large countries, such as the USA or Canada, even have multiple time zones. Interestingly, even though China is large enough to have multi time zones, she uses only one time zone. This sometimes results in such an awkward situation where the sun rises around 10:00 AM in the western part of China

GMT, UTC, and Offset


The Korean local time is normally GMT +09:00. GMT is an abbreviation for Greenwich Mean Time, which is the clock time at the Royal Observatory in Greenwich, U.K. located at longitude 0. The GMT system began spreading in Feb. 5, 1925 and became the world time standard until Jan. 1, 1972.


Many consider GMT and UTC the same thing, and the two are used interchangeably in many cases, but they are actually different. UTC was established in 1972 to compensate for the slowing problem of the Earth’s rotation. This time system is based on International Atomic Time, which uses the cesium atomic frequency to set the time standard. In other words, UTC is the more accurate replacement system of GMT. Although the actual time difference between the two is tiny, UTC is whatsoever the more accurate choice for software developers.


+09:00 in UTC+09:00 means the local time is 9 hours ahead than the UTC standard time. This means that it’s 09:00 PM in Korea when it’s 12:00 PM in a UTC region. The difference of time between UTC standard time and the local time is called “offset”, which is expressed in this way: +09:00, -03:00, etc.

Time zone !== offset?

As I mentioned earlier, we use the names of time zones (KST, JST) interchangeably with offset without distinguishing them. But it’s not right to treat the time and offset of a certain region the same for the following reasons:

Summer Time (DST)

Although this term might be unfamiliar to some countries, a lot of countries in the world adopted summer time. “Summer time” is a term mostly used in the U.K. and other European countries. Internationally, it is normally called Daylight Saving Time (DST). It means advancing clocks to one hour ahead of standard time during summer time.

Does Time Zone Changes?

As I briefly mentioned earlier, each country has its own right to determine which time zone to use, which means its time zone can be changed due to any political and/or economic reasons. For example, in the states, the period of DST was changed in 2007 because President George Bush signed the energy policy in 2005. Egypt and Russia used to use DST, but they ceased to use it since 2011.

Time Zone 1 : Offset N

To summarize, a time zone can have one or more offsets. Which offset a country will use as its standard time at a certain moment can vary due to political and/or economic reasons.

IANA Time Zone Database

To tell you the truth, time zones are more of a database rather than a collection of rules because they must contain all relevant historical changes. There are several standard database designed to handle the time zone issues, and the most frequently used one is IANA Time Zone Database. Usually called tz database (or tzdata), IANA Timezone Database contains the historical data of local standard time around the globe and DST changes. This database is organized to contain all historical data currently verifiable to ensure the accuracy of time since the Unix time (1970.01/01 00:00:00). Although it also has data before 1970, the accuracy is not guaranteed.

JavaScript and IANA Time Zone Database

As I briefly mentioned earlier, the time zone feature of JavaScript is quite poor. Since it follows the time zone of the region by default (to be more specific, the time zone selected at the time of the OS installation), there is no way to change it to a new time zone. Also, its specifications for database standard are not even clear, which you will notice if you take a close look at the specification for ES2015. Only a couple of vague declarations are stated regarding local time zone and DST availability. For instance, DST is defined as follows: ECMAScript 2015 — Daylight Saving Time Adjustment

Time Zone in Server-Client Environment

We will assume a simple scenario in which time zone must be considered. Let’s say we’re going to develop a simple calendar app that will handle time information. When a user enters date and time in the field on the register page in the client environment, the data is transferred to the server and stored in the DB. Then the client receives the registered schedule data from the server to displays it on screen.

Date Object in JavaScript

In JavaScript, tasks involving date or time are handled using a Date object. It is a native object defined in ECMAScript, like Array or Function. which is mostly implemented in native code such as C++. Its API is well described in MDN Documents. It is greatly influenced by Java’s java.util.Date class. As a result, it inherits some undesirable traits, such as the characteristics of mutable data and month beginning with 0.

Creating Date Object with User Input

Let’s go back to the first example. Assume that a user entered 11:30 AM, Mar 11, 2017 in a device which follows the time zone of Seoul. This data is stored in 5 integers of 2017, 2, 11, 11, and 30 — each representing the year, month, day, hour, and minute, respectively. (Since the month begins with 0, the value must be 3–1=2.) With a constructor, you can easily create a Date object using the numeric values.

const d1 = new Date(2017, 2, 11, 11, 30);
d1.toString(); // Sat Mar 11 2017 11:30:00 GMT+0900 (KST)

Creating Date Object Using Server Data

Let’s now assume that you are going to receive data from the server. If the data is of the numerical Unix time value, you can simply use the constructor to create a Date object. Although I skipped the explanation earlier, when a Date constructor receives a single value as the only parameter, it is recognized as a Unix time value in millisecond. (Caution: JavaScript handles Unix time in milliseconds. This means that the second value must be multiplied by 1,000.) If you see the example below, the resultant value is the same as that of the previous example.

const d1 = new Date(1489199400000);
d1.toString(); // Sat Mar 11 2017 11:30:00 GMT+0900 (KST)
const d1 = new Date('2017-03-11T11:30:00');
const d2 = new Date('2017-03-11T11:30:00Z');
d1.toString(); // "Sat Mar 11 11:30:00 UTC+0900 2017"
d2.toString(); // "Sat Mar 11 20:30:00 UTC+0900 2017"

Creating Data to be Transferred to Server

Now use the Date object created earlier, and you can freely add or subtract time based on local time zones. But don’t forget to convert your data back to the previous format at the end of the processing before transferring it back to the server.

const d1 = new Date(2017, 2, 11, 11, 30);
d1.getTime(); // 1489199400000
const d1 = new Date(2017, 2, 11, 11, 30);
d1.toISOString(); // "2017-03-11T02:30:00.000Z"
d1.toJSON(); // "2017-03-11T02:30:00.000Z"

const d2 = new Date('Hello');
d2.toISOString(); // Error: Invalid Date
d2.toJSON(); // null

Changing Local Time Zone

You can see now that JavaScript provides a bit of support for time zone. What if you want to change the local time zone setting within your application without following the time zone setting of your OS? Or what if you need to display a variety of time zones at the same time in a single application? Like I said several times, JavaScript does not allow manual change of local time zone. The only solution to this is adding or removing the value of the offset from the date provided that you already know the value of the time zone’s offset. Don’t get frustrated yet though. Let’s see if there is any solution to circumvent this.

const seoul = new Date(1489199400000);
seoul.getTimeZoneOffset(); // -540
function formatDate(date) {
return date.getFullYear() + '/' +
(date.getMonth() + 1) + '/' +
date.getDate() + ' ' +
date.getHours() + ':' +

const seoul = new Date(1489199400000);
const ny = new Date(1489199400000 - (840 * 60 * 1000));

formatDate(seoul); // 2017/3/11 11:30
formatDate(ny); // 2017/3/10 21:30

Problem of Converting Local Time Zone

If you keep working with the example above a little more, you will soon face with a problem. The user wants to check the time in New York local time and then change the date from 10th to 15th. If you use the setDate() method of Date object, you can change the date while leaving other values unchanged.

formatDate(ny); // 2017/3/15 21:30
const time = ny.getTime() + (840 * 60 * 1000);  // 1489631400000
const time = ny.getTime() + (780 * 60 * 1000);  // 1489627800000

Moment Timezone

Moment is a well established JavaScript library that is almost the standard for processing date. Providing a variety of date and formatting APIs, it is recognized by so many users recently as stable and reliable. And there is Moment Timezone, an extension module, that solves all the problems discussed above. This extension module contains the data of IANA Time Zone Database to accurately calculate offsets, and provides a variety of APIs that can be used to change and format time zone.

const seoul = moment(1489199400000).tz('Asia/Seoul');
const ny = moment(1489199400000).tz('America/New_York');

seoul.format(); // 2017-03-11T11:30:00+09:00
ny.format(); // 2017-03-10T21:30:00-05:00

seoul.date(15).format(); // 2017-03-15T11:30:00+09:00
ny.date(15).format(); // 2017-03-15T21:30:00-04:00


So far, we’ve discussed the time zone APIs supported by JavaScript and their issues. If you don’t need to manually change your local time zone, you can implement the necessary features even with basic APIs provided that you’re using Internet Explorer 9 or higher. However, if you need to manually change the local time zone, things get very complicated. In a region where there is no summer time and time zone policy hardly changes, you can partially implement it using getTimezoneOffset() to convert the data. But if you want full time zone support, do not implement it from scratch. Rather use a library like Moment Timezone.




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