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Issues With Converting Epoch Time to Correct Date

I have files with epoch time stamps such as 1564002293050. Using https://www.epochconverter.com/ this shows Wednesday, July 24, 2019 9:04:53.050 PM however my code shows Mon Apr 06 51531 02:24:10 GMT-0700 (Pacific Daylight Time). Why is this?
Because the times were generated in Unix, I multiplied by 1000 for ms. This value is then displayed.
time = 1564002293050;
var dateStamp = new Date(time* 1000);
Edit:
Ive referenced this post and several similar others. It is good to note that not multiplying it by 1000 will result with "Invalid Date".
Edit 2:
Figure it out. I was parsing the data but looks like I had to convert it to an integer parseInt(time) ended up fixing the issue. Sorry for the unrelated solution..

document.write(new Date(1564002293050));
Prints Wed Jul 24 2019 15:04:53 GMT-0600 (Mountain Daylight Time) (in my TZ).
Even this outputs the same in HTML, despite not syntactically declaring said variable.
time = 1564002293050;
var dateStamp = new Date(time);
document.write(dateStamp);
Are you doing this on the browser? Another Javascript engine?

The Date constructor takes the epoch time in milliseconds (https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Date). Your time is in milliseconds. No need to multiply it by 1000.

.NET date, Moment.js, UTC and Timezone shifting

I've got a UTC date through an Ajax call, e.g. "/Date(1517216466000+0100)/",
which is when printing to the console: Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time).
What I need to do, is to let the user change the timezones: I can easily do it with e.g. moment(myDate).tz("Japan").
Then I need to save the date in it's UTC format, which I am not able to do.
I've been experimenting with moment.utc(), but for the input above, it returns 1 hour less.
How to deal with this situation? In summary:
1. Get a UTC time from a webservice
2. Let the user change the timezones
3. Save the modified date in UTC (without the timezone)
Working demo: https://stackblitz.com/edit/angular-kqrct7?file=app%2Fapp.component.html
EDIT for clarification:
Let's just have a look at the hours. What I get the date from the WCF, is 10 o'clock. The browser interprets it as 10 o'clock BUT in GMT+1 so when I convert it to UTC, it becomes 9 o'clock.
I want it to be 10 o'clock as UTC. Then, if I modify the timezone and e.g. the minutes of this date, I want to be able to get the UTC value of this date.
EDIT2: Made my question simplier for clarification
I've got a UTC date, which I get from a webservice like: "/Date(1517216466000+0100)/" which is: Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time) when printed to console.
I add a timezone to it with moment(this.inputDate).tz("Europe/Berlin").format(), but it stays 10:01:06, I guess because of my browsers GMT+1.
I want the ORIGINAL string to be used as a UTC date AND it should remain 10:01:06, not 09:01:06 as you can see above (2nd moment example), so with the timezone "Europe/Berlin" would be 11:01:6

In the .NET JSON formatted date "/Date(1517216466000+0100)/" the timezone offset can be ignored. It represents "2018-01-29T09:01:06.000Z", where the source system was at a timezone offset of +0100. So if you don't care about the source timezone, just ignore it.
It is also an identical moment in time to Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time), just with a different offset.
UTC is not a format, it's a time standard. If you want to use ISO 8601 format:
Extract the first numeric value
Convert to Number
Pass to the Date constructor
Call the toISOString method on the resulting Date
var s = '/Date(-1517216466000+0100)/';
console.log(new Date(+s.replace(/^[^\d-]+(-?\d+).*$/,'$1')).toISOString());
You can also parse and format it using moment.js, which according to the documentation can handle the .NET JSON format without needing to specify the format. So you can either do that or extract the time value and parse it with the "x" format token:
var s = '/Date(1517216466000+0100)/';
// Let moment.js guess the format
console.log(moment(s).utc());
// Extract time value and supply format
console.log(moment(s.replace(/^[^\d-]+(-?\d+).*$/,'$1'), 'x').utc());
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.20.1/moment.min.js"></script>

"/Date(1517216466000+0100)/" is a non-standard way to serialise a date/time. Take a look at ISO8601 which defines several standard ways to represent dates and times.
That being said let's take a look at what this gets evaluated as...
moment("/Date(1517216466000+0100)/").toDate()
gives Mon Jan 29 2018 09:01:06 GMT+0000 (GMT Standard Time) for me (in the UK)
taking just the timestamp value 1517216466000
new Date(1517216466000)
also gives Mon Jan 29 2018 09:01:06 GMT+0000 (GMT Standard Time)
this means that the +0100 is being ignored.
You're not actually modifying the time so why would you expect it to save back as anything other than Mon Jan 29 2018 09:01:06
UPDATE
But the "original" string represents Mon Jan 29 2018 09:01:06 UTC the +0100 is ignored and it's just coincidence that your offset to UTC is also +0100. An off.
Offset and timezone are 2 different things. A timezone encompasses offset to UTC as well as when/if daylight savings comes in to force. Just because it says +0100 doesn't necessarily mean (W. Europe Standard Time) as it could just as easily be (West Africa Time) which is also UTC+0100 but doesn't observe daylight savings at all.
The time you have "/Date(1517216466000+0100)/" doesn't convey enough information to say which timezone it is and JS/moment just uses the timestamp 1517216466000 and as such uses UTC. When you console.log() this the browser writes it to screen as local time Mon Jan 29 2018 10:01:06 GMT+0100 (W. Europe Standard Time) but this is only a representation of the underlying datetime.
By telling moment to use a specific time zone, it's only changing how the date/time gets displayed and doesn't actually change the time it represents.
If you use a date picker to change the date/time then you'll have to serialise the value to send to the backend in an appropriate way that the .Net app you cannot change will understand and conveys what you intend.
Example
Get date from server as var serverTime = "/Date(1517216466000+0100)/"
convert this to a JS Date using moment var time = new moment(serverTime)
Let user specify TimeZone i.e. Japan Standard Time (UTC+9) time = time.tz("Japan")
time still represents Mon Jan 29 2018 09:01:06 UTC but when displayed on screen with time.format() gives "2018-01-29T18:01:06+09:00"
You stated "I want it to be 10 o'clock as UTC". Unfortunately the value you've is NOT 10 O'clock UTC and will never be 10 O'clock UTC because it isn't. It is 9 O'clock UTC
You could parse the value you get from the server yourself but the time from the server IS 9am UTC. If you change it to 10 am UTC then you would see that as Mon Jan 29 2018 11:01:06 GMT+0100 (W. Europe Standard Time) - 11 O'clock local time.

Thanks everyone for your detailed answers, they helped me a lot in understanding my problem! However, the right solution was the following:
10 o'clock was a UTC time in the database and it got interpreted as 9 o'clock UTC in Moment.js because C# handled it as a local time. So, before sending the date to the client, I had to indicate that its a UTC:
var utcToClient = DateTime.SpecifyKind(downtime.DownTimeStartUTC, DateTimeKind.Utc)
Then, in Moment I could create a UTC with:
var jsUtc = moment.utc(downtime.DownTimeStartUTC)
Changing the timezones was a breeze with:
jsUtc.tz(userSelectedTimezone)
And saving the date in the database, I used this in C#:
var utcFromClient = Record.DownTimeStartUTC.ToUniversalTime()

Wrong date with angular material's date picker

I use the datepicker to pick a date and send it to the server.
When I log the JS value I get the correct result:
Tue Mar 22 2016 00:00:00 GMT+0100 (Mitteleuropäische Zeit)
but in the ajax request it is
2016-03-21T23:00:00.000Z
I don't modify the values, just giving the object to angulars http function.
Does Angular need some configuration to handle it?

You can try the following piece of code
dateObj.setMinutes((dateObj.getMinutes() + dateObj.getTimezoneOffset()));
No need of localization, use this code just before doing any service call. It will pass you the exact date what you selected in the datepicker.
It will work in all timezone (+) and (-),
Example: 2016-03-21 00:00:00 GMT+0100, the above said code covert it as 2016-03-21 01:00:00 GMT+0000. While on Service it converts it as 2016-03-21 00:00:00.
I think it will solve your problem.

Those two strings represent the same time. One is in UTC, i.e. GMT +0, which you can see from the Z ending. The other is in a different timezone, specifically GMT +1 hour.
If you had javascript date objects for both strings, and turned them into integers, i.e. seconds passed since Jan 1, 1970, UTC, you'd find them identical. They represent the same instant but in two different geographic locations.
var d1 = new Date('Tue Mar 22 2016 00:00:00 GMT+0100');
var d2 = new Date('2016-03-21T23:00:00.000Z');
Number(d1); // 1458601200000
Number(d2); // 1458601200000
Generally this is a good thing. Dealing in timezones gets very confusing. I find it best for a server to always deal in UTC.

https://github.com/angular/material/pull/9410
Check out the 1.1.1+ version. This will solve your issue.
<md-datepicker ng-model="date" ng-model-options="{ timezone: 'utc' }"></md-datepicker>

If suppose am selecting a date like Tue Aug 06 2019 00:00:00 GMT+0530 (India Standard Time), am getting 2019-08-05T18:30:00.000Z. ( which in my case previous date with respect to the selected date)
I made use of toLocalDateString() to do the job.
// this.date = 2019-08-05T18:30:00.000Z
const converted = new Date(this.date).toLocaleDateString();
console.log(converted); // 8/6/2019 (MM/DD/YYYY) format

what does the T separator in JavaScript Date

I think I need clarification on something:
I have a string representing a date in a format like this:
'2013-12-24 12:30:00'
and if I pass it to Date(), then I get the following output
new Date('2013-12-24 12:30:00')
// --> Tue Dec 24 2013 12:30:00 GMT+0100
because iOS has problems with this, I read that I should use T as separator, however
new Date('2013-12-24T12:30:00')
// --> Tue Dec 24 2013 13:30:00 GMT+0100
the result adds one hour. I guess it has something to do with summer or winter, but what exactly does the T stand for, and why is the result different? I meanwhile solved my problem by passing separate parameters to the Date but I would still like to know where this extra hour is coming from.

new Date('2013-12-24T12:30:00')
treats the time as UTC, so it's 12:30 in Greenwich and 13:30 in your timezone.
new Date('2013-12-24 12:30:00')
is a Chrome extension (or bug) that doesn't work in other browsers. It treats the time as local, so it's 12:30 in your timezone (GMT+1) and 11:30 in Greenwich.

If you look closely... you will notice adding a T makes the time in 24-hour format.
new Date('2013-12-24T12:30:00')
// --> Tue Dec 24 2013 13:30:00 GMT+0100
Compared to
new Date('2013-12-24 12:30:00')
// --> Tue Dec 24 2013 12:30:00 GMT+0100

I guess its stands for Long time pattern. Refer this for more.

DateJS parsing mystery

I'm using DateJS to parse user-inputted dates, and getting some strange results.
Date.parse("15 Jan 2010") returns Fri Jan 15 00:00:00 EST 2010 (right)
Date.parse("15-Apr-2010") returns Thu Apr 15 00:00:00 EDT 2010 (right)
Date.parse("15 Apr 2010") returns Thu Apr 1 00:00:00 EDT 2010 (wrong)
As far as I can tell, the d MMM yyyy input format works fine for every month except April and August; in those two cases, it returns the first of the month no matter what day is entered. Is this a bug, or is there a logical explanation I'm missing?

Aha: Looks like the version in the "Download" link is a good bit older than the current source. Here's the commit that fixed this bug:
Dan Yoder fixed bug with timeContext pattern where if a date included
"april" or "august", the parser thought the 'a' was the beginning of a time part
(as in am/pm).
The most recent version of the EN-US script is here:
http://code.google.com/p/datejs/source/browse/trunk/build/date-en-US.js
It would be nice if the website linked to this instead of to a zip file that hasn't been updated for a couple of years.