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Converting Timestamp String with Timezone into UTC in Javascript Without External Packages

I have a timestamp string and a timezone, and I need to convert that into a Date object in UTC without using moment-timezone.
Essentially, I wanna be able to do this without external dependencies:
var date = moment.tz("2021-03-03 14:40:40", "Asia/Dhaka")
Is this possible? I'd rather not have to download a rather hefty package just for this.

Check it out
let dateString = new Date("2021-03-03 14:40:40").toDateString('en-US', {timeZone: 'Asia/Dhaka'})
console.log(dateString);

How to convert into Date time

When the date is passed from my c# to JavaScript it returns the date time as {4/3/2020 12:00:00 AM}
but in JavaScript it is shown as 1585852200000.
What is the format that is being used? And how can i convert it back?

You need to convert the Unix timestamp to DateTime format,
var localDate = new Date(1585852200000).toLocaleDateString("en-US")
console.log(localDate); // only local date
var localTime = new Date(1585852200000).toLocaleTimeString("en-US")
console.log(localTime) // only local time
// local datetime
console.log(new Date(1585852200000).toLocaleString());

1585852200000 is epoch date.
you can convert it as
var date = new Date(1585852200000)
console.log(new Date(1585852200000));

As an alternative from Shivaji's answer:
When you are passing the date through to JS you could cast it as a string with DateTime.ToString("dd/MM/yyyy") seen here on MSDN.
This will keep its integrity visually, if it is just for display purposes, otherwise you will need to re-cast appropriately in JS (in which case use Shivaji's answer).

JavaScript Date's object will return the DATE object and it's POSITION that is being assigned in your computer. So, when you are working with a date or datetime types, you can use some of the methods that are provided by the Date object, such as getDate() and getDay(). But, a better solution would be to format the Date object itself. For example: use the toString() or toUTCString() methods.
var d = new Date();
document.getElementById("demo").innerHTML = d.toString();
Reference:
https://www.w3schools.com/js/js_date_formats.asp
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date

how to get only time from date in ISO formate

How to extract only time from the date which is present in ISO format?
I tried this:
var d = new Date('1970-01-15T03:32:12.000Z'); //ISO-8601 formatted date returned from server
console.log(d.getTime());// 1222332000
Expected op is : 03:32:12

Since your server returns an ISO-8601 formatted date which has a predefined format, you can convert it to ISO string using toISOString() and then get the substring of the time value:
var d = new Date('1970-01-15T03:32:12.000Z');
console.log(d.toISOString().substr(11,8));

Date.getTime() returns the time in UNIX epoch format.
https://en.wikipedia.org/wiki/Unix_time
To access only the parameters you are interested in, you can use Date.getMinutes(), Date.getMinutes(), etc. See docs on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date
Note: Do not forget to spend one thought on time zones when you work with Date
's time, especially when your app runs in different regions.

You have to manually build the time string using Date.prototype methods: getHours, getMinutes and getSeconds
Or use moment.js library.

Date.getTime() gives you the unix timestamp, which is the number of seconds since january 1st 1970;
The getTime() method returns the numeric value corresponding to the time for the specified date according to universal time.
from MDN
You need to format the date yourself, either by concatenating the output of the Date.getHours(), Date.getMinutes() and Date.getSeconds() methods, or by using one of the predefined formatting functions, like Date.toTimeString(). Checkout the docs to pick your choice.

You can use getHours(),getMinutes() and getSecondes(). Then you can use it with strings or objects.

Try the following:
d.toTimeString().split(' ')[0]

You can use moment.js to parse whatever format you like.
If you think moment.js is too big, there's another library call dayjs. The same fashion API but just 2KB. (Unfortunately, you can't do UTC time with dayjs yet.)
Update: Thanks kun for notifying the updates. You can now use UTC with dayjs plugin since v1.8.9.
var d = new Date('1970-01-15T03:32:12.000Z');
console.log(moment(d).utc().format('HH:mm:ss'));
dayjs.extend(dayjs_plugin_utc)
console.log(dayjs(d).utc().format('HH:mm:ss'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.2/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.8.9/dayjs.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.8.9/plugin/utc.js"></script>

Convert UTC time to local time zone in IE in javascript

I have a date time string in format ("2015-10-07 15:20:00 UTC") and i want to convert it to local time zone of client. i am using the following statements for this:
var UTC_Time = new Date ("2015-10-07 15:20:00 UTC");
var localTime = UTC_Time.toString();
in Google Chrome it works fine and return the converted time as 2015-10-07 20:20:00 PST which is fine. But in internet explorer (i am concerned with IE10) it is returning the same UTC date i.e. 2015-10-07 15:20:00. how can i get the converted time in IE. Any help would be greatly appreciated.

When you display a date in javascript, it converts it to the client time. Since you are specifying UTC in your date string, it will assume that it's a UTC date. There are a couple ways you can solve this.
If you just need a string, you can do localTime = UTC_Time.toUTCString().
If you need a js Date object, you can create a new date object by getting the values from the previous object.
new Date(UTC_Time.getUTCFullYear(), UTC_Time.getUTCMonth(),
UTC_Time.getUTCDate(), UTC_Time.getUTCHours(), UTC_Time.getUTCMinutes(),
UTC_Time.getUTCSeconds(), UTC_Time.getUTCMilliseconds());
Or you can simply replace the UTC part of the string.
var dtStr = "2015-10-07 15:20:00 UTC";
dtStr = dtStr.replace(" UTC", "");
var localTime = new Date(dtStr);
Only use this option if you know your string will always be in the same format.

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