Handling Time Zone in JavaScript

What is Time zone?

GMT, UTC, and Offset

When the system was still in development, anglophones wanted to name the system CUT (Coordinated Universal Time) and francophones wanted to name it TUC (Temps Universal Coordonn). However, none of the either side won the fight, so they came to an agreement of using UTC instead, as it contained all the essential letters (C, T, and U).

The entire list of UTC offsets and their names can be found in List of UTC Time offsets.

Time zone !== offset?

Time Zone 1 : Offset N

IANA Time Zone Database

Note that Windows is not in the above support list. It’s because Windows uses its own database called Microsoft Time Zone Database. However, this database does not accurately reflect historical changes and managed only by Microsoft. Therefore, it is less accurate and reliable than IANA.

An implementation dependent algorithm using best available information on time zones to determine the local daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds. An implementation of ECMAScript is expected to make its best effort to determine the local daylight saving time adjustment.

NOTE : It is recommended that implementations use the time zone information of the IANA Time Zone Database http://www.iana.org/time-zones/.

Time Zone in Server-Client Environment

Even when you’re working with JavaScript in a server environment using Node.js, you might have to parse the data retrieved from the client depending on the case. However since servers normally have their time zone synced to the database and the task of formatting is usually left to clients, you have fewer factors to consider than in a browser environment. In this article, my explanation will be based on the browser environment.

Date Object in JavaScript

As I mentioned earlier, JavaScript does not provide any arbitrary way to change time zone. Therefore, I will assume a situation here where the time zone setting of the browser can be directly used.

Creating Date Object with User Input

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

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

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

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

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

Some of you may wonder why I added using the converted data when I have to convert it back anyway before returning. It looks like I can just process it without conversion and temporarily create a converted Date object only when I’m formatting. However, it is not what it seems. If you change the date of a Date object based on Seoul time from 11th to 15th, 4 days are added (24 * 4 * 60 * 60 * 1000). However, in New York local time, as the date has been changed from 10th to 15th, resultantly 5 days have been added (24* 5 * 60 * 60 * 1000). This means that you must calculate dates based on the local offset to get the precise result.

const time = ny.getTime() + (780 * 60 * 1000);  // 1489627800000

On the contrary, if the user’s local time zone is New York and wants to know the time in Seoul, DST is applied unnecessarily, causing another problem.

Moment Timezone

In this article, I won’t discuss how to use library or the structure of library in details. I will just show you how simple it is to solve the problems I’ve discussed earlier. If anyone is interested, see Moment Timezone’s Document.

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





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