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Convert JavaScript Date to formatted String

I have a JavaScript Date object and want to convert it into String like this:
2018-05-24T11:00:00+02:00
var dateObj = new Date("Thu May 24 2018 11:00:00 GMT+0200");
function convertToString(dateObj) {
// converting ...
return "2018-05-24T11:00:00+02:00";
}

You can use moment.js, it handles pretty much all the needs about date formatting you may have.
var dateObj = new Date("Thu May 24 2018 11:00:00 GMT+0200");
console.log(moment(dateObj).format())

You have quite the options to represent the DateTime object as a string. This question was already elaborated on in the following StackOverflow answers:
Using toLocaleDateString()
Using dateFormat library (requires the use of external library)
Vanilla JavaScript, adds a few extra lines, but the format is entirely up to you
Personally, I would sacrifice a few extra lines in my document for the Vanilla JavaScript variant. This way I would have complete control of the format and of the function responsible for the formatting - easier debugging and future changes. In your case that would be (using string literals to shorten the code):
var date = new Date("Thu May 24 2018 11:00:00 GMT+0200");
function convertToString(date) {
return `${date.getFullYear()}-${date.getMonth()}-${date.getDate()}-...`;
}
And so on. On this page Date - JavaScript | MDN, in the left you have all the methods that extract some kind of information from the Date object. Use them as you wish and you can achieve any format you desire. Good luck!

How to get hours in moment.js in GMT timezone?

For example if I do for the above date object something like: value.hours(), I get as output 16 instead of 18. I believe it returns the hours in the original GMT time, not like in my date object which is GMT+2. I can of course add 2 to the returned result, but it becomes cumbersome. Is there any way to get the hours correctly in my case?

I'm not sure as to what you've already tried, but I put the following into JSFiddle and it worked like a charm. I am currently in CST in America and it is 8:30 in the morning here. When I ran the snippet below I got today's date at 1:30 PM which I would assume is accurate in difference.
HTML
<div id="m1"></div>
JavaScript
var a = moment.tz(new Date(), "GMT");
document.getElementById('m1').innerHTML = a.format("YYYY MM DD; HH:mm");
The Moment.js documentation states the following in regards to creating a Moment object with a native JavaScript Date object:
You can create a Moment with a pre-existing native JavaScript Date object.
var day = new Date(2011, 9, 16);
var dayWrapper = moment(day);
This clones the Date object; further changes to the Date won't affect the Moment, and vice-versa.
To find the information quoted above quickly, when you reach the Moment.js documentation, it is located under the Parse section under sub-section Date.

To display local time:
value.local();
value.hours(); // 18
To reverse:
value.utc();
value.hours(); // 16

I think that you can solve it by doing what the docs says. Something like this:
moment().tz("America/Los_Angeles").format();
https://momentjs.com/timezone/docs/#/using-timezones/

Javascript: how to convert a UTC date to local one?

Looks like a very simple thing to do? Not afrer reading this http://dygraphs.com/date-formats.html - what a mess!
var time_utc="2016-04-25 20:19:00.306671";
document.write("Local date:"+new Date(time_utc+" UTC")); // Firefox 44.0.2: Invalid Date
How do I print a date in above format adjusted to local time?

The article you provided mentions halfway through the page,
Using hyphens (-) instead of slashes (/) works in WebKit browsers, but
not in IE or FF. Beware the UTC parse of YYYY-MM-DD!
As well as,
Don't try to specify milliseconds. Only recent versions of Chrome will
understand you.
With the date 2016-04-25 20:19:00.306671 you use hyphens and milliseconds. You could modify your time_utc string a bit to make it compatible like so,
var time_utc = "2016-04-25 20:19:00.306671";
time_utc = time_utc.replace(/-/g, "/");
time_utc = time_utc.split(".").shift();
var d = new Date(time_utc);
d.toString();
The above code outputs,
Mon Apr 25 2016 20:19:00 GMT+0200 (CEST)

Have you looked into Moment.js? http://momentjs.com/ It's a handy date-object wrapper that makes date object manipulation easy. Particularly, the local() function provided will give you what you need here.
All you have to do is install moment from npm and then include it in your js file at the top like this:
var moment = require("moment");
Then to change your time_utc variable to local all you have to do is:
var time_utc="2016-04-25 20:19:00.307";
document.write("Local date:"+moment(time_utc).local());
As someone advised me before, it is not wise to include an entire library for a simple, one time function. As a disclaimer, my work requires me to do many date-time calculations and conversions throughout a large project, so including a library for ease is much preferred in my case. However, if you only have to use it this one time, my answer may not be the best.

If you use moment.js, you can use:
var time_utc = "2016-04-25 20:19:00.306671";
var localDate = moment.utc(time_utc).local();

You need append UTC to the string before converting it to a local date in js:
var date = new Date('25/04/2016 4:52:48 PM UTC');
date.toString() // "Mon Apr 25 2016 09:52:48 GMT-0700 (PDT)"

How do I convert to browser timezones using phstc dateFormat?

EDIT : Btw, I have no idea why this question was marked as a duplicate. The answers in the original question does not work for me. i.e, getting wrong results and stuffs. Furthermore, none of the answers deal with phstc's dateFormat function. Do correct me if I'm wrong. Btw, I have solved this question. Do take a look at my answer.
I want to change a UTC datetime to my browser's timezone. I'm using phstc's dateFormat in pure javascript form. Let's say I convert a datetime of 2014-06-27 07:11:16 using a javascript Date() function. The result I got was
Fri Jun 27 2014 07:11:16 GMT+0800 (Malay Peninsula Standard Time)
Then when I use phstc's toBrowserTimeZone function, it still returns me the same datetime. I wanted to get something like 2014-06-27 15:11:16
Here is the code below:
var originalDateTime = new Date(`2014-06-27 07:11:16`);
alert(DateFormat.format.toBrowserTimeZone(originalDateTime,"yyyy/MM/dd HH:mm:ss"));

According to this statement in phstc's dateFormat page,
value = String representing date in ISO time (“2013-09-14T23:22:33Z”) or String representing
default JAXB formatting of java.util.Date (“2013-09-14T16:22:33.527-07:00”) or String representing
Unix Timestamp (Sat Sep 14 2013 16:22:33 GMT-0700 (PDT)) or javascript date object.
JS Date object should work but unfortunately, it didn't. Well, I got it fixed by changing the datetime to other formats stated above first before calling the toBrowserTimeZone() function. For example,
var originalDateTime = DateFormat.format.date('2014-06-27 07:11:16',"yyyy-MM-ddTHH:mm:ssZ");
var newDateTime = DateFormat.format.toBrowserTimeZone(originalDateTime);

Date format conversion javascript

Consider if we have a date in the format Sat Jul 28 2012 , is there a general a function to convert it in to any wanted format??
say for example 28-07-2012,
deciding the separators like - or /

Javascript's Date object has lots of different versions of toString and different getters, so it should be pretty easy to get the output you want. Scroll down through this documentation to see some of your options. They have pretty good examples too if you click on them.
In addition, the Date constructor is fairly good at taking in most strings and converting it.
var myDate = new Date("Sat Jul 28 2012");
alert(myDate.toLocaleDateString());
Or use the different getters and string concatenation wrapped in a function to make your own.

You can write a function to do it, i dont think there is a Native method:
function convert(dateObj) {
var format = dateObj.getFullYear()+"-";
format += dateObj.getMonth()+"-";
format += dateObj.getDate();
return format;
}
you can customize it however you want. here is the list of methods for the date object

Javascript only outputs it into the standard format you provided above. You can try using the getDate(), getDay(), getMonth() methods (among others) to extract the necessary data and convert it to your liking.
Please refer to W3Schools' description of the JavaScript Date object.