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How to display times in different time zones using the offset value

I am trying to display the current moment in different time zones. I have tried to use native javascript and the moment-js package but it seems you require the time zone name (ex. "America/Toronto") to display the information I want. The problem is that the information I currently have is the timestamp in string format (see below for string format) from the desired timezone, and the city the timestamp belongs to. The problem with using the city to create my tz value is that one of the cities I want to display isn't in the IANA tz database (Calgary).
String timestamp:
2022-04-26T14:19:42.8430964-04:00
As can be seen I do have the time offset and I was hoping there was a way to convert this string in js to a Date object and then display the time using the offset in the correct time zone.
Note what I want to display is the following format:
12:19 pm MT
I know I could just parse out the string but I feel like this isn't the best practice, however I may be wrong.
Also note that I will have to get the time zone (ex. MT, ET it can also be MST, EST) using the offset.

you don't need moment here. JS can do this natively. Also moment is now deprecated.
Your IANA timezone is America/Edmonton. So it's simple to do. That date format is an ISO 8601 date format, so you can just pass it into the new Date constructor and it'll parse it correctly:
const iso8601 = '2022-04-26T14:19:42.8430964-04:00';
const date = new Date(iso8601);
console.log(date.toLocaleString('en-US', { timeZone: 'America/Edmonton' }));
It doesn't matter what timezone your input date is set, so long as it has the correct offset in the ISO date. Once it's a Date, the offset is irrelevant. E.g.:
const iso8601 = '2022-04-26T14:19:42.8430964Z';
const date = new Date(iso8601);
//time will now be 4 hours off above as the input date is UTC
console.log(date.toLocaleString('en-US', { timeZone: 'America/Edmonton' }));

convert date based on timezone user is in

I have a date I want to convert based on their timezone.
For example, I want to set it in EST time (America/New_York)
2019-04-24 12:00:00
and if the user comes across the site from America/Los_Angeles, it will appear:
2019-04-24 09:00:00
I need to be able to return the hour, so in that example: 9.
I tried using https://github.com/iansinnott/jstz to determine their timezone and https://moment.github.io/luxon in hopes of handling the conversion w/o any luck.
I was testing by changing the timezone on my computer w/o any luck.

It sounds like you're asking to convert from a specific time zone to the user's local time zone (whatever it may be). You do not need time zone detection for that, but at present you do need a library. (Answers that suggest using toLocaleString with a time zone parameter are incorrect, as that function converts to a specific time zone, but cannot go the other direction.)
Since you mentioned Luxon, I'll provide a Luxon specific answer:
luxon.DateTime.fromFormat('2019-04-24 12:00:00', // the input string
'yyyy-MM-dd HH:mm:ss', // the format of the input string
{ zone: 'America/New_York'}) // the time zone of the input
.toLocal() // convert to the user's local time
.toFormat('yyyy-MM-dd HH:mm:ss') // return a string in the same format
//=> "2019-04-24 09:00:00"
This capability is also provided by other libraries, such as date-fns-timezone, js-Joda, or Moment-Timezone, but it is not yet something built in to JavaScript.

Converting date based on the time can be done like this. reference convert date to another time zone example snippet is under.
var usaTime = new Date().toLocaleString("en-US", {timeZone: "America/New_York"});
usaTime = new Date(usaTime);
console.log('USA time: '+usaTime.toLocaleString())
var usaTime = new Date().toLocaleString("en-US", {timeZone: "America/Los_Angeles"});
usaTime = new Date(usaTime);
console.log('USA time: '+usaTime.toLocaleString())

You could keep a list of timzeone identifiers and a list of their corresponding +/- number of hours with respect to your local time (which is returned by your time function).
Once you have a user's time zone, and you have extracted the current hour from the local timestamp simply look up the timezone in your list and use it's index to access the second list to find how many hours to add or subtract from the users time.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleString
var date = new Date(Date.UTC(2012, 11, 12, 3, 0, 0));
// toLocaleString() without arguments depends on the implementation,
// the default locale, and the default time zone
console.log(date.toLocaleString());
// → "12/11/2012, 7:00:00 PM" if run in en-US locale with time zone America/Los_Angeles
Or you can use getYear, getMonth, getDate, getHours, getMinutes, getSeconds to format your own representation of the date. These methods all return values according to the user's local timezone.

I think the question may need more clarification - my first impression was you refer to a date-time that you already have and serve from the server. Doesn't this problem boil down to the Date object being "user-timezone-aware"? or not? But it is (some methods are, to be exact)
Your date/time is 2019-04-24 12:00:00 EDT (i assume P.M.)
This means the Unix timestamp of this in milliseconds is 1556121600000
(i assume daylight is on for April so not pure EST but EDT and an offset of UTC-4:00)
When you call
console.log(new Date(1556121600000).getHours())
doesn't this return 9 as you suggest, for Javascript executed on a browser from America/Los_Angeles with PDT timezone?
As suggested at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getHours :
The getHours() method returns the hour for the specified date,
according to local time.

Working with different timezones in Javascript

I am working on a cloud based application which deals extensively with date and time values, for users across the world.
Consider a scenario, in JavaScript, where my machine is in India (GMT+05:30), and I have to display a clock running in California's timezone (GMT-08:00).
In this case I have to get a new date object,
let india_date = new Date()
add it's time zone offset value,
let uts_ms = india_date.getTime() + india_date.getTimezoneOffset()
add california's timezone offset value,
let california_ms = utc_ms + getCaliforniaTimezoneOffsetMS()
and finally the date object.
let california_date: Date = new Date(california_ms)
Is there any way to directly deal with these kinds of time zones without having to convert the values again and again?

First, let's talk about the code in your question.
let india_date = new Date()
You have named this variable india_date, but the Date object will only reflect India if the code is run on a computer set to India's time zone. If it is run on a computer with a different time zone, it will reflect that time zone instead. Keep in mind that internally, the Date object only tracks a UTC based timestamp. The local time zone is applied when functions and properties that need local time are called - not when the Date object is created.
add it's timezone offset value
let uts_ms = india_date.getTime() + india_date.getTimezoneOffset()
This approach is incorrect. getTime() already returns a UTC based timestamp. You don't need to add your local offset. (also, the abbreviation is UTC, not UTS.)
Now add california's timezone offset value
let california_ms = utc_ms + getCaliforniaTimezoneOffsetMS()
Again, adding an offset is incorrect. Also, unlike India, California observes daylight saving time, so part of the year the offset will be 480 (UTC-8), and part of the year the offset will be 420 (UTC-7). Any function such as your getCaliforniatimezoneOffsetMS would need to have the timestamp passed in as a parameter to be effective.
and finally the date object
let california_date: Date = new Date(california_ms)
When the Date constructor is passed a numeric timestamp, it must be in terms of UTC. Passing it this california_ms timestamp is actually just picking a different point in time. You can't change the Date object's behavior to get it to use a different time zone just by adding or subtracting an offset. It will still use the local time zone of where it runs, for any function that requires a local time, such as .toString() and others.
There is only one scenario where this sort of adjustment makes sense, which is a technique known as "epoch shifting". The timestamp is adjusted to shift the base epoch away from the normal 1970-01-01T00:00:00Z, thus allowing one to take advantage of the Date object's UTC functions (such as getUTCHours and others). The catch is: once shifted, you can't ever use any of the local time functions on that Date object, or pass it to anything else that expects the Date object to be a normal one. Epoch shifting done right is what powers libraries like Moment.js. Here is another example of epoch shifting done correctly.
But in your example, you are shifting (twice in error) and then using the Date object as if it were normal and not shifted. This can only lead to errors, evident by the time zone shown in toString output, and will arise mathematically near any DST transitions of the local time zone and the intended target time zone. In general, you don't want to take this approach.
Instead, read my answer on How to initialize a JavaScript Date to a particular time zone. Your options are listed there. Thanks.

JavaScript Date objects store date and time in UTC but the toString() method is automatically called when the date is represented as a text value which displays the date and time in the browser's local time zone. So, when you want to convert a datetime to a time zone other than your local time, you are really converting from UTC to that time zone (not from your local time zone to another time zone).
If your use case is limited to specific browsers and you are flexible on formatting (since browsers may differ in how they display date string formats), then you may be able to use toLocaleString(), but browsers like Edge, Android webview, etc do not fully support the locales and options parameters.
Following example sets both the locale and timezone to output the date in a local format that may vary from browser to browser.
const dt = new Date();
const kolkata = dt.toLocaleString('en-IN', { timeZone: 'Asia/Kolkata' });
const la = dt.toLocaleString('en-US', { timeZone: 'America/Los_Angeles' });
console.log('Kolkata:', kolkata);
// example output: Kolkata: 19/3/2019, 7:36:26 pm
console.log('Los Angeles:', la);
// example output: Los Angeles: 3/19/2019, 7:06:26 AM
You could also use Moment.js and Moment Timezone to convert date and time to a time zone other than your local time zone. For example:
const dt = moment();
const kolkata = dt.tz('Asia/Kolkata').format();
const la = dt.tz('America/Los_Angeles').format();
console.log(kolkata);
// example output: 2019-03-19T19:37:11+05:30
console.log(la);
// example output: 2019-03-19T07:07:11-07:00
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.24.0/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment-timezone/0.5.23/moment-timezone-with-data.min.js"></script>

Well, you really do kind of have to convert anytime you want to change the display, but it's not as bad as you think.
First, store all time as UTC. Probably using the milliseconds format, e.g. Date.UTC().
Second, do all manipulation / comparison using that stored info.
Third, if your cloud-based application has an API that API should only talk in terms of UTC as well, though you could provide the ISO string if you prefer that to the MS, or if you expect clients to handle that better.
Fourth and finally, only in the UI should you do the final conversion to the local date/time string, either with the method you're describing or using a library such as momentjs

new Date creates a Date object with a time value that is UTC. If you can guarantee support for the timeZone option of toLocaleString (e.g. corporate environment with a controlled SOE), you can use it to construct a timestamp in any time zone and any format, but it can be a bit tedious. Support on the general web may be lacking. A library would be preferred in that case if you need it to work reliably.
E.g. to get values for California, you can use toLocaleString and "America/Los_Angeles" for the timeZone option:
var d = new Date();
// Use the default implementation format
console.log(d.toLocaleString(undefined, {timeZone:'America/Los_Angeles'}));
// Customised format
var weekday = d.toLocaleString(undefined, {weekday:'long', timeZone:'America/Los_Angeles'});
var day = d.toLocaleString(undefined, {day:'numeric', timeZone:'America/Los_Angeles'});
var month = d.toLocaleString(undefined, {month:'long', timeZone:'America/Los_Angeles'});
var year = d.toLocaleString(undefined, {year:'numeric', timeZone:'America/Los_Angeles'});
var hour = d.toLocaleString(undefined, {hour:'numeric',hour12: false, timeZone:'America/Los_Angeles'});
var minute = d.toLocaleString(undefined, {minute:'2-digit', timeZone:'America/Los_Angeles'});
var ap = hour > 11? 'pm' : 'am';
hour = ('0' + (hour % 12 || 12)).slice(-2);
console.log(`The time in Los Angeles is ${hour}:${minute} ${ap} on ${weekday}, ${day} ${month}, ${year}`);
Getting the timezone name is a little more difficult, it's difficult to get it without other information.

momentjs toDate() date return not same as format() date

I am using moment 2.16.0 and want starting days of month. There are different result of toDate() and format() method. Here is jsfiddle.
code:-
var time=moment().subtract(0,'months').startOf("month").format();
console.log(time); //2016-12-01T00:00:00+05:30
var time2=moment().subtract(0, 'months').endOf("month").format();
console.log(time2); //2016-12-31T23:59:59+05:30
var time=moment().subtract(0,'months').startOf("month").toISOString();
console.log(time); //2016-11-30T18:30:00.000Z here i want somethings like 2016-12-01T00:00:00.000Z
var time2=moment().subtract(0, 'months').endOf("month").toISOString();
console.log(time2); // 2016-12-31T18:29:59.999Z here i want somethings like 2016-12-31T59:59:59.000Z

All of your operations are using moment with local time except the toISOString, which will give you the string in UTC. Since your timezone is offset from UTC, naturally the local time string (from format) and the UTC time string (from toISOString) are very different.
here i want somethings like 2016-12-01T00:00:00.000Z
That would be a different time from what that Moment instance represents.
If you want something in an ISO-8601 format but in local time, you can use format with the appropriate set of formatting tokens, but you don't wan the Z at the end because, again, you're not dealing with UTC ("Zulu") time, you're dealing with local time.
moment().format("YYYY-MM-DDThh:mm:ss.SSS")

Convert string to new Date object in UTC timeZONE

Can anyone let me know how to convert a string to a date Object with UTC time zone in ExtJs?
String is "2015-10-07T23:59:00". I would like to get the same in Date Object without changing the timezone.

First of all, your date string does not have a timezone.
When you make a JavaScript date object from a string, there are two possible outcomes you could expect:
You may want the date to be 23:59 Local (23:59 CEST in my case).
In this case, you want to use new Date("2015-10-07 23:59:00") with plain javascript (note the missing T), or Ext.Date.parse("2015-10-07T23:59:00","c");.
You may want the date to be 23:59 UTC (e.g. 01:59 CEST).
In this case, you want to use new Date("2015-10-07T23:59:00").
Of course, whenever you output the date, you have to get the date in the correct time zone as well. The console/toString will usually show it in local time. JavaScript does provide getUTC... methods if you require other time zones.
You see, using Time Zones with JavaScript is a painful experience. I would recommend to try moment.js if you need full time zone support.

You can use Ext.Date.parse.It gives Date Object as output.It syntax is:
Ext.Date.parse( String input, String format, [Boolean strict] )
For Example:
Ext.Date.parse("2015-10-07T23:59:00", "Y-m-dTH:i:s");

try
var millisFromEpoch = Date.parse('2015-10-07T23:59:00');
it will parse date in GMT timezone, Ext.date.parse use the current timezone instead