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Convert date with timezone to UTC?

A quick question. I have a ISO string date:
2022-07-03T10:51:09+02:00
this date as you can see has timezone included (+02:00).
Question: How to convert it into UTC date? Using e.g. date-fns or moment?
Edit: Should I just simply add "02:00" hours to current date? So it would be 12:51:09?

Trivially new Date(isoString).toISOString(), no libraries required.
const input = "2022-07-03T10:51:09+02:00";
console.log(`${input} in UTC:\n${new Date(input).toISOString()}`);

In my view, timezone is simply the representation of the same timestamp across different geographies (no. of seconds elapsed since unix time 0 is the same everywhere). So be careful while adding/removing time manually from the existing timestamp.
You can do that using moment.js like this:
var someday = moment('2022-07-03T10:51:09+02:00');
console.log(someday.utc().format());
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.29.4/moment-with-locales.min.js"></script>

Alternative to casting UTC Date in Javascript?

I wish to create a new Date in JS, but have it be cast as UTC time. For example, suppose castAsUTC() produces the following desired effect:
var x = new Date('2019-01-01T00:00:00') // In local time (PST)
castAsUTC(x).toISOString(); // => '2019-01-01T00:00:00Z'
// x.toISOString() gives us '2019-01-01T08:00:00Z', which is undesired
Currently, my function looks like this:
function castAsUTC(date) {
return new Date(x.toLocaleString() + '+00:00');
}
Is there a cleaner/nicer way of producing the same effect? Thanks in advance!
EDIT: To be more specific, I'm interested in transforming the date's timezone, without changing its actual value with as little arithmetic as possible. So calling .toISOString() will produce the same date as it is in local time.
I am currently using the moment-timezone library, but I can't seem to get the desired effect using that, either. I would definitely accept an answer that uses Moment.js

You can switch a Moment instance to UTC using the utc function. Then just use format to get whatever the specific output you want from it.

If indeed the string you have is like the one shown, then the easiest thing to do would be to append a Z to indicate UTC.
var input = '2019-01-01T00:00:00';
var date = new Date(input + 'Z');
var output = date.toISOString();
Or, if you would like to use Moment.js, then do this:
var input = '2019-01-01T00:00:00';
var m = moment.utc(input);
var output = m.format();
You do not need moment-timezone for this.

tl;dr;
You formatted the date wrong. Add the letter "Z" to the end of your date string and it will be treated as UTC.
var x = new Date('2019-01-01T00:00:00Z') // Jan 1, 2019 12 AM UTC
These formatting issues are easier to manage with a library like momentjs (utc and format functions) as described in other answers. If you want to use vanilla javascript, you'll need to subtract out the timezone offset before calling toISOString (see warnings in the longer answer below).
Details
Date in javascript deals with timezones in a somewhat counter intuitive way. Internally, the date is stored as the number of milliseconds since the Unix epoch (Jan 1, 1970). That's the number you get when you call getTime() and it's the number that's used for math and comparisons.
However - when you use the standard string formatting functions (toString, toTimeString, toDateString, etc) javascript automatically applies the timezone offset for the local computers timezone before formatting. In a browser, that means it will apply the offset for the end users computer, not the server. The toISOString and toUTCString functions will not apply the offset - they print the actual UTC value stored in the Date. This will probably still look "wrong" to you because it won't match the value you see in the console or when calling toString.
Here's where things really get interesting. You can create Date's in javascript by specifying the number of milliseconds since the Unix epoch using new Date(milliseconds) or by using a parser with either new Date(dateString). With the milliseconds method, there's no timezone to worry about - it's defined as UTC. The question is, with the parse method, how does javascript determine which timezone you intended? Before ES5 (released 2009) the answer was different depending on the browser! Post ES5, the answer depends on how you format the string! If you use a simplified version of ISO 8601 (with only the date, no time), javascript considers the date to be UTC. Otherwise, if you specify the time in ISO 8601 format, or you use a "human readable" format, it considers the date to be local timezone. Check out MDN for more.
Some examples. I've indicated for each if javascript treats it as a UTC or a local date. In UTC, the value would be Jan 1, 1970 at midnight. In local it depends on the timezone. For OP in pacfic time (UTC-8), the UTC value would be Jan 1, 1970 at 8 AM.
new Date(0) // UTC (milliseconds is always UTC)
new Date("1/1/1970"); // Local - (human readable string)
new Date("1970-1-1"); // Local (invalid ISO 8601 - missing leading zeros on the month and day)
new Date("1970-01-01"); // UTC (valid simplified ISO 8601)
new Date("1970-01-01T00:00"); // Local (valid ISO 8601 with time and no timezone)
new Date("1970-01-01T00:00Z"); // UTC (valid ISO 8601 with UTC specified)
You cannot change this behavior - but you can be pedantic about the formats you use to parse dates. In your case, the problem was you provided an ISO-8601 string with the time component but no timezone. Adding the letter "Z" to the end of your string, or removing the time would both work for you.
Or, always use a library like momentjs to avoid these complexities.
Vanilla JS Workaround
As discussed, the real issue here is knowing whether a date will be treated as local or UTC. You can't "cast" from local to UTC because all Date's are UTC already - it's just formatting. However, if you're sure a date was parsed as local and it should really be UTC, you can work around it by manually adjusting the timezone offset. This is referred to as "epoch shifting" (thanks #MattJohnson for the term!) and it's dangerous. You actually create a brand new Date that refers to a different point in time! If you use it in other parts of your code, you can end up with incorrect values!
Here's a sample epoch shift method (renamed from castAsUtc for clarity). First get the timezone offset from the object, then subtract it and create a new date with the new value. If you combine this with toISOString you'll get a date formatted as you wanted.
function epochShiftToUtc(date) {
var timezoneOffsetMinutes = date.getTimezoneOffset();
var timezoneOffsetMill = timezoneOffsetMinutes * 1000 * 60;
var buffer = new Date(date.getTime() - timezoneOffsetMill);
return buffer;
}
epochShiftToUtc(date).toUTCString();

Convert ISO Date string to date in JavaScript

Need help to convert exactly from ISO Date string to Date:
I have an ISO string date: "2016-01-23T22:23:32.927".
But when I use new Date(dateString) to convert Date, the result is wrong:
var date = new Date("2016-01-23T22:23:32.927");
The result is: Sun Jan 24 2016 05:23:32 GMT+0700. It's not true. I want the date is 23 not 24.
Please help me. Thanks a lot!

You need to supply a timezone offset with your iso date. Since there isn't one, it assumes the date to be in GMT and when you log it out, it prints it in the timezone of your browser. I think that if you pass "2016-01-23T22:23:32.927+07:00" to new Date() you would get the value you are expecting.

JavaScript environments (browser, node,...) use a single timezone for formatting dates as strings. Usually this is your system's timezone. Based on the output you get, yours is GMT+0700.
So what happened:
The string you passed as ISO format to the Date constructor doesn't specify a timezone. In this case it is treated as UTC.
When you then output the date (I'll assume with console.log), it is converted to the timezone of your environment. In this case 7 hours where added.
If that doesn't suit you, you can change the way you output the date. This depends on what output you want, e.g.:
If you just want the UTC timezone again, you can use date.toISOString().
If you want to output it in another timezone, you can call date.getTimezoneOffset() and figure out the difference between both timezones. You'd then probably need to get the individual date parts and add/subtract the timezone difference accordingly. At this point you could consider using an existing library, taking into account their possible disadvantages.

If you're willing and able to add a dependency, I recommend using moment.js for this. It makes date handling in Javascript much more straightforward and a lot safer, and fixes your specific problem right out of the box.
To do this, 1st load it from a CDN, e.g. Moment.JS 2.14.1 minified. Then use it as follows:
var date = moment("2016-01-23T22:23:32.927");
console.log(date);
// output: Sat Jan 23 2016 22:23:32 GMT-0500
...i.e. your desired result :)
Here's a jsfiddle demonstrating this.

Use date.toUTCString()
it'll give you 23 instead of 24 as it Convert a date object to a string, according to universal time

How to parse date string from database without time shift?

I am getting 2016-07-13T00:00:00.000Z string from database and converting it to MM/DD/YYYY format with moment.js like this:
result = moment('2016-07-13T00:00:00.000Z').format('MM/DD/YYYY');
which prints 07/12/2016 but I was expecting 07/13/2016.
Local Linux timezone is America/New_York. date command prints this Mon Jul 4 04:28:19 EDT 2016

The date that you have is in UTC, as signified by the z at the end.
When you use the default moment constructor, moment(), it converts the time you pass it from the specified offset (in this case UTC) to the local time of the machine. This is why your date is changing. Because this is a UTC date, to keep it exactly the same you can use moment.utc():
moment.utc('2016-07-13T00:00:00.000Z').format('MM/DD/YYYY');
"07/13/2016"
Alternately, parseZone would work as well:
moment.parseZone('2016-07-13T00:00:00.000Z').format('MM/DD/YYYY');
"07/13/2016"
For more information about all of the constructor functions in moment, see the parsing guide
or this blog post

You should use momentjs timezone : http://momentjs.com/timezone/
yourdate = moment.tz("2016-07-13T00:00:00.000Z", "America/New_York");
alert(yourdate.format('MM/DD/YYYY'));
This should give you the correct date in output.

moment.js does not convert timestamps with specific timezones to unix timestamps

I am using moment.js to convert a bunch of timestamps in it's specific timezone to a unix timestamp like this:
var timestamp = "2015-12-29T09:35:00.000-08:00";
console.log(moment("2015-12-29T09:35:00.000-08:00").unix();
console.log(moment("2015-12-29T09:35:00.000-08:00").tz("America/Los_Angeles").unix();
The console log of both the above statements is for some reason, the same - 1451361900. This unix timestamp which it is logging is in my local timezone and not the one I asked for: "America/Los_Angeles". What am I missing?

A unix timestamp, or Posix, should always be in the UTC (Coordinated Universal Time) format.
Moment is just doing something like
function unix () {
return Math.floor(+this / 1000);
}
Where it converts the date object to an integer and then converts from milliseconds to seconds.
The starting point is a regular javascript Date object, and the ECMA standard says
Date objects are based on a time value that is the number of
milliseconds since 1 January, 1970 UTC.
so date objects are always UTC when converted to the number of milliseconds since 1. January 1970 (epoch), i.e. you can't set another timezone on a Unix timestamp, both your dates are the same.

The proper way is to use moment-Timezone is this.
console.log(moment("2015-12-29T09:35:00").unix());
console.log(moment.tz("2015-12-29T09:35" , "America/Los_Angeles").unix());
In above your are providing time zone as a string too which is this last part ".000-08:00" and then you are providing another zone, which is incorrect.

As you are trying to find out the unix timestamp for the date "2015-12-29T09:35:00.000-08:00". In this date format timezone value is already present which is "-08:00", hence you get the same unix timestamp.
For getting the unix timestamp desired solution, remove the timezone value and use moment-timezone as :
console.log(moment.tz("2013-12-01", "America/Los_Angeles").unix());
For more details check moment-timezone