Take my timezone as an example (AEST):

function parseDate(str_date) {
  return new Date(Date.parse(str_date));
}


var str_date = "2015-05-01T22:00:00+10:00"; //AEST time
var locale_date = parseDate(str_date);

locale_date: Fri May 01 2015 22:00:00 GMT+1000 (AEST)

var str_date = "2015-05-01T22:00:00+00:00" //UTC time
var locale_date = parseDate(str_date);

locale_date: Sat May 02 2015 08:00:00 GMT+1000 (AEST)

Here is the one-liner:

function convertTZ(date, tzString) {
    return new Date((typeof date === "string" ? new Date(date) : date).toLocaleString("en-US", {timeZone: tzString}));   
}

// usage: Asia/Jakarta is GMT+7
convertTZ("2012/04/20 10:10:30 +0000", "Asia/Jakarta") // Tue Apr 20 2012 17:10:30 GMT+0700 (Western Indonesia Time)

// Resulting value is regular Date() object
const convertedDate = convertTZ("2012/04/20 10:10:30 +0000", "Asia/Jakarta") 
convertedDate.getHours(); // 17

// Bonus: You can also put Date object to first arg
const date = new Date()
convertTZ(date, "Asia/Jakarta") // current date-time in jakarta.

   

This is the MDN Reference.

Beware the caveat: function above works by relying on parsing toLocaleString result, which is string of a date formatted in en-US locale , e.g. "4/20/2012, 5:10:30 PM". Each browser may not accept en-US formatted date string to its Date constructor and it may return unexpected result (it may ignore daylight saving).

Currently all modern browser accept this format and calculates daylight saving correctly, it may not work on older browser and/or exotic browser.

side-note: It would be great if modern browser have toLocaleDate function, so we don't have to use this hacky work around.

Background

JavaScript's Date object tracks time in UTC internally, but typically accepts input and produces output in the local time of the computer it's running on. It has very few facilities for working with time in other time zones.

The internal representation of a Date object is a single number - namely timestamp - representing the number of milliseconds that have elapsed since 1970-01-01 00:00:00 UTC, without regard to leap seconds.

There is no time zone or string format stored in the Date object itself.

When various functions of the Date object are used, the computer's local time zone is applied to the internal representation. If the function produces a string, then the computer's locale information may be taken into consideration to determine how to produce that string. The details vary per function, and some are implementation-specific.

The only operations the Date object can do with non-local time zones are:

• It can parse a string containing a numeric UTC offset from any time zone. It uses this to adjust the value being parsed, and stores the UTC equivalent. The original local time and offset are not retained in the resulting Date object. For example:

    var d = new Date("2020-04-13T00:00:00.000+08:00");
    d.toISOString()  //=> "2020-04-12T16:00:00.000Z"
    d.valueOf()      //=> 1586707200000  (this is what is actually stored in the object)
  

• In environments that have implemented the ECMASCript Internationalization API (aka "Intl"), a Date object can produce a locale-specific string adjusted to a given time zone identifier. This is accomplished via the timeZone option to toLocaleString and its variations. Most implementations will support IANA time zone identifiers, such as 'America/New_York'. For example:

    var d = new Date("2020-04-13T00:00:00.000+08:00");
    d.toLocaleString('en-US', { timeZone: 'America/New_York' })
    //=> "4/12/2020, 12:00:00 PM"
    // (midnight in China on April 13th is noon in New York on April 12th)
  

  Most modern environments support the full set of IANA time zone identifiers (see the compatibility table here). However, keep in mind that the only identifier required to be supported by Intl is 'UTC', thus you should check carefully if you need to support older browsers or atypical environments (for example, lightweight IoT devices).

Libraries

There are several libraries that can be used to work with time zones. Though they still cannot make the Date object behave any differently, they typically implement the standard IANA timezone database and provide functions for using it in JavaScript. Modern libraries use the time zone data supplied by the Intl API, but older libraries typically have overhead, especially if you are running in a web browser, as the database can get a bit large. Some of these libraries also allow you to selectively reduce the data set, either by which time zones are supported and/or by the range of dates you can work with.

Here are the libraries to consider:

Intl-based Libraries

New development should choose from one of these implementations, which rely on the Intl API for their time zone data:

• Luxon (successor of Moment.js)
• date-fns-tz (extension for date-fns)
• Day.js (when using its Timezone plugin)

Non-Intl Libraries

These libraries are maintained, but carry the burden of packaging their own time zone data, which can be quite large.

• js-joda/timezone (extension for js-joda)
• moment-timezone* (extension for Moment.js)
• date-fns-timezone (extension for older 1.x of date-fns)
• BigEasy/TimeZone
• tz.js

* While Moment and Moment-Timezone were previously recommended, the Moment team now prefers users chose Luxon for new development.

Discontinued Libraries

These libraries have been officially discontinued and should no longer be used.

• WallTime-js
• TimeZoneJS

Future Proposals

The TC39 Temporal Proposal aims to provide a new set of standard objects for working with dates and times in the JavaScript language itself. This will include support for a time zone aware object.

Common Errors

There are several approaches that are often tried, which are in error and should usually be avoided.

Re-Parsing

new Date(new Date().toLocaleString('en', {timeZone: 'America/New_York'}))

The above approach correctly uses the Intl API to create a string in a specific time zone, but then it incorrectly passes that string back into the Date constructor. In this case, parsing will be implementation-specific, and may fail entirely. If successful, it is likely that the resulting Date object now represents the wrong instant in time, as the computer's local time zone would be applied during parsing.

Epoch Shifting

var d = new Date();
d.setTime(d.getTime() + someOffset * 60000);

The above approach attempts to manipulate the Date object's time zone by shifting the Unix timestamp by some other time zone offset. However, since the Date object only tracks time in UTC, it actually just makes the Date object represent a different point in time.

The same approach is sometimes used directly on the constructor, and is also invalid.

Epoch Shifting is sometimes used internally in date libraries as a shortcut to avoid writing calendar arithmetic. When doing so, any access to non-UTC properties must be avoided. For example, once shifted, a call to getUTCHours would be acceptable, but a call to getHours would be invalid because it uses the local time zone.

It is called "epoch shifting", because when used correctly, the Unix Epoch (1970-01-01T00:00:00.000Z) is now no longer correlated to a timestamp of 0 but has shifted to a different timestamp by the amount of the offset.

If you're not authoring a date library, you should not be epoch shifting.

For more details about epoch shifting, watch this video clip from Greg Miller at CppCon 2015. The video is about time_t in C++, but the explanation and problems are identical. (For JavaScript folks, every time you hear Greg mention time_t, just think "Date object".)

Trying to make a "UTC Date"

var d = new Date();
var utcDate = new Date(Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), d.getUTCHours(), d.getUTCMinutes(), d.getUTCSeconds(), d.getUTCMilliseconds()));

In this example, both d and utcDate are identical. The work to construct utcDate was redundant, because d is already in terms of UTC. Examining the output of toISOString, getTime, or valueOf functions will show identical values for both variables.

A similar approach seen is:

var d = new Date();
var utcDate = new Date(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), d.getUTCHours(), d.getUTCMinutes(), d.getUTCSeconds(), d.getUTCMilliseconds());

This is approach passes UTC values into the Date constructor where local time values are expected. The resulting Date object now represents a completely different point in time. It is essentially the same result as epoch shifting described earlier, and thus should be avoided.

The correct way to get a UTC-based Date object is simply new Date(). If you need a string representation that is in UTC, then use new Date().toISOString().

I want to combine these strings into one datetime object like (2020-05-05T15:30:00-09:00)

Date objects are extremely simple, they're just a time value that is an offset in milliseconds since 1970-01-01T00:00:00Z, so are inherently UTC. The built–in parser is unreliable and lacks any functionality such as format tokens.

So if you have separate values like:

• Date string (2020-05-05)
• Time string (15:30)
• Timezone offset (-09:00)

then you can create a string that is compliant with the format defined in ECMA-262 and that should be parsed correctly by the built–in parser, e.g.

new Date('2020-05-05T15:30:00.000-09:00')

However, general advice is to avoid the built–in parser due to differences in implementations. Also, the format must be exact (e.g. including seconds and milliseconds in the timestamp, colon (:) in the offset) or some implementations will reject it as malformed and return an invalid date.

Once you have a Date object, getting a "local" timestamp with offset is an issue of formatting, which has been answered many times before (e.g. How to format a JavaScript date). There aren't any decent built–in formatting functions (toLocaleString with options is OK for some purposes but generally lacking in functionality), so you'll have to either write your own function, or use a library.

The following examples use Luxon, which is suggested as the upgrade path from moment.js.

With Luxon, if you specify a representative location, you'll get the offset for that location at the date's date and time. Alternatively, you can fix the offset to a set value, essentially setting it for a timezone without a representative location, so it doesn't have any reference to daylight saving or historic offset changes:

let DateTime = luxon.DateTime;

// Offset per the specified location
let d0 = DateTime.fromISO('2020-01-01', {zone: 'America/Yakutat'});
let d1 = DateTime.fromISO('2020-06-30', {zone: 'America/Yakutat'});
console.log(d0.toString());
console.log(d1.toString());

// Fixed offset per the supplied string
let d2 = DateTime.fromISO('2020-05-05T15:30:00.000-09:00', { setZone: true});
let d3 = DateTime.fromISO('2020-01-01T15:30:00.000-09:00', { setZone: true});
console.log(d2.toString());
console.log(d3.toString());
  

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