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Convert date from UK GMT string to local date in JavaScript

I have a situation where I am always returned the date from the server as a UK date time string.
E.g. '2020-07-19 16:40:00'
This would be 4:40PM in UK at +01:00, or 3:40PM UTC.
I want to be able to convert this time from GMT to the local time on the computer;
If I do this when in the UK...
var date = new Date('2020-06-19 16:40:00 GMT');
it returns Fri Jun 19 2020 17:40:00 GMT+0100 (British Summer Time)
Which is an hour out.
If I do the date in winter time (without daylight savings), this is correct.
var date = new Date('2020-01-19 16:40:00 GMT');
returns Sun Jan 19 2020 16:40:00 GMT+0000 (Greenwich Mean Time)
Is there a way I can correctly adjust this to always give the correct time regardless of what timezone the computer is set in, based on UK clock times.
Thanks in advance

As I understand your question, you don't know if the timestamp from the server is GMT or BST as the offset isn't included. You can work it out using plain JS but it's somewhat kludgy and error prone, see Calculate Timezone offset only for one particular timezone.
It would be much better to get the server to use an ISO 8601 format supported by ECMAScript and either send the offset or always use UTC/GMT.
If that isn't an option, you can use a library like Luxon to specify the location (and hence offset rules) to use for parsing, e.g.
let DateTime = luxon.DateTime;
['2020-07-19 16:40:00', // BST +1
'2020-01-19 16:40:00' // GMT +0
].forEach(ts => console.log(
DateTime.fromFormat(ts, 'yyyy-LL-dd HH:mm:ss', {zone: 'Europe/London'}))
);
<script src="https://cdn.jsdelivr.net/npm/luxon#1.24.1/build/global/luxon.min.js"></script>
PS Don't forget to always tell the parser the format to parse.

Object property looking different after printing on it's own

I am trying to use luxon to generate a new date using a timezone. This is my code:
var luxon = require('luxon');
luxon.Settings.defaultZoneName = 'UTC+4';
var date = luxon.DateTime.local();
console.log(date);
var now = new Date(date.ts);
console.log(now.toString());
And this is the console:
DateTime {
ts: 2018-09-13T13:09:45.333+04:00,
zone: UTC+4,
locale: en-US }
Thu Sep 13 2018 11:09:45 GMT+0200 (CEST)
But if I try to access the ts property like so
var date = luxon.DateTime.local();
console.log(date.ts); // here
var now = new Date(date.ts);
console.log(now.toString());
I get this in the console:
1536830052009
Thu Sep 13 2018 11:14:12 GMT+0200 (CEST)
Why is that? Is it doing some kind of math in the background? Also it turns out this date.ts is just ignoring my timezone. How can I fix that?

First 1536830052009, This is your time in milliseconds,
new Date(1536830052009)
// output Thu Sep 13 2018 11:14:12 GMT+0200 (CEST)
You may want to check your timezone with getTimezoneOffset()
Returns the time difference between UTC time and local time, in minutes
Many people use moment.js to play with Date, I know it is not in your question but maybe you could find some usefull things

ts is not a public property and you shouldn't use it. Luxon does all sorts of tricks under the covers to get the math right. If you want the timestamp, just use date.toMillis(). If you want a JS Date, use date.toJSDate().
Two other important things to know:
It's not ignoring your zone. The zone doesn't change the time. It's more like metadata about a time that affects how we display it. The Luxon docs cover this a bit. You shouldn't expect to extract a different timestamp by fiddling with the zone. Now is always now.
Remember that the native Date object doesn't support timezones other than your local one. So anytime you convert from a Luxon object to a native Date, that information is lost. The time itself will be the same (meaning, it will represent the same millisecond), but it will express it in the local time.

How do I deal with dates-only in javascript when it keeps appending a time value?

I have a bunch of date fields (not datetime) in SQL Server. When they are fetched by the web server and sent to the client as JSON a time stamp is appended automatically. So instead of receiving just 2016-09-27 I get 2016-09-27T00:00:00.
When the user interacts with the uiBootstrap calendar control it automatically parses that string into a javascript date object and applies a 4 hour offset for the timezone. When this is sent back to the server it's sent as 2016-09-26T20:00:00. Now my date is off by a day. Also the next time it's fetched it will happen again. But this time it will start at 2016-09-26T00:00:00 and will roll back to 2016-09-25T20:00:00. Each cycle between client and server loses a day.
How do I keep my dates from changing? I'm looking at moment.js but so far haven't really figured out how it can help me.
EDIT
I've setup a test function to try different methods of converting datetimes back and forth.
console.log('JSONDate: ' + JSONDate);
var dt = new Date(JSONDate);
console.log('JS Converted Date: ');
console.log(dt);
console.log('Date converted back to string: ' + dt.toISOString());
Here's the output:
JSONDate: 2016-10-02T00:00:00
JS Converted Date: Sun Oct 02 2016 00:00:00 GMT-0400 (Eastern Daylight Time)
Date converted back to string: 2016-10-02T04:00:00.000Z
In this example the date is now 4 hours ahead.
EDIT 2
Web server is running .net, specifically WebAPI 2. I'm using Entity Framework 6 to communicate between web server and SQL Server 2012.

Ideally, your dates would be serialized in the JSON as just dates. Instead of 2016-10-02T00:00:00, you'd have 2016-10-02. The problem is that .NET doesn't have a built in Date type. It only has DateTime. There are alternatives, such as LocalDate in Noda Time, as discussed in this answer.
However, assuming you don't want to change anything on the back-end, the way to handle this is just to make sure the input date/time is treated as local time, and never converted to/from UTC. This should be the default behavior when you parse the string into a Date object when the string is like 2016-10-02T00:00:00, but the behavior has changed a few times over the years, so if you are potentially dealing with older browsers, you may get some that interpret it as UTC instead.
As far as output goes, the toISOString method of the Date object always outputs in UTC - which is the source of your conversion error. If you want an ISO8601 string in local time - you'd have to construct one yourself using the various accessor functions (getFullYear, etc.), handling zero-padding, and ensuring months are incremented to be 1-based instead of 0-based.
The easier solution is to use moment.js, which can handle this for you.
var d = moment('2016-10-02T00:00:00').toDate(); // now you have a `Date` object
var s = moment(d).format("YYYY-MM-DD[T]HH:mm:ss"); // now you have a string again
Of course, if you don't need the time portion, you can omit it from the format string and the rest should still work out ok.

You could try getting the offset and applying it back to the date. Something like this:
var d = new Date('2016-09-27'); //Mon Sep 26 2016 20:00:00 GMT-0400 (EDT)
new Date(d.getTime() + d.getTimezoneOffset() * 60 * 1000) //Tue Sep 27 2016 00:00:00 GMT-0400 (EDT)

How to get browser time zone in JavaScript

Is it possible to get the user time zone in JavaSript in tz database format e.g. America/Los Angeles?
I noticed that when I create a Date object its toString method returns something like this:
Thu May 08 2014 08:40:48 GMT-0700 (Pacific Standard Time)
so maybe it's possible to change it to one of ids in tz database from this region (unfortunately chrome shows Pacific Daylight Time so it seems like there's no standard format to display). I need some library because I don't have time to right it myself with all possible combination and discrepancies in browsers like firefox vs chrome.

Best answer I found so far
var d = new Date()
var gmtHours = -d.getTimezoneOffset()/60;
console.log("The local time zone is: GMT " + gmtHours);

From: http://www.w3schools.com/jsref/jsref_gettimezoneoffset.asp
Return the timezone difference between UTC and Local Time:
var d = new Date()
var n = d.getTimezoneOffset();
The result n will be 420. Divide that by 60 to difference in hours.
You could then create an array populated text so that:
timezoneArray[n] = "tz text description";

How to update the date to the current date for UTC -8

I want to display a UTC date using this JavaScriptcode on my webpage.
<script>
function myDate()
{
var now = new Date();
var d = new Date(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate());
var x = document.getElementById("demo");
x.innerHTML=d;
}
</script>
With this code I am getting UTC date displayed as a local string as follows: "Thu Jul 04 2013 00:00:00 GMT+0530 (India Standard Time)"
I do not want display the string with a local time offset (GMT+0530 (IST)), instead I want the time to appear as UTC string format

The date returned by different browser are of different format
to remove GMT OFFSET from date you can use replace
var d = new Date(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate());
d = d.toString().replace(/GMT.+/,"");

Firstly, the problem is that you are instantiating a local Date object by passing in the UTC year, month and day. This then creates a local Date with the values provided. by doing this you might be creating an incorrect date based on whether you want it to be UTC or local. IN your case, if you want var now as UTC, the way you are currently instantiating is incorrect as its in local time.
Anyway, dates can be tricky in in JavaScript, so I would consider using Moment.js for this
It's a fantastic library that provides all of the functions for manipulating and converting JavaScript dates that you could ever need.
For example with moment you can just do the following:
var now = moment(); // current date and time in local format
var nowAsUTC = now.utc(); // current local date and time converted to UTC
var alsoNowAsUTC = moment.utc() // same as the line above, but staring in UTC
console.log(nowUTC.format("DD/MM/YYYY, hh:mm:ss"))// prints a pretty UTC string

Hmmm.. Are you sure you want to display UTC-8? I will take a guess that you are really wanting to convert the time to US Pacific time zone. That is not always UTC-8. Sometimes it is UTC-8, and sometimes it is UTC-7.
If you're not actually in the US Pacific Time zone, the only way to do this reliably in JavaScript is with a library that implements the TZDB database. I list several of them here.
For example, using walltime-js library, you can do the following:
var date = new Date();
var pacific = WallTime.UTCToWallTime(date, "America/Los_Angeles");
var s = pacific.toDateString() + ' ' + pacific.toFormattedTime();
// output: "Fri Apr 26 2013 5:44 PM"
You can't just add or subtract a fixed number, because the target time zone may use a different offset depending on exactly what date you're talking about. This is primarily due to Daylight Saving Time, but also because time zones have changed over time.