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Sitemap lastmod date in Javascript

I need to the date format that is shown in <lastmod></lastmod> tags in sitemap.xml. How can I achieve that?
Output like:
<lastmod>2016-01-21EEST18:01:18+03:00</lastmod>
Where 'EEST' is probably timezone offset.
I have seen this question and the answers in php:
Google Sitemap Date Format
but don't know how to achieve it in Javascript,
Thanks.
EDIT: the question is not duplicate, since I need the correct time format in JavaScript for this.

The lastmod tag uses YYYY-MM-DDThh:mmTZD format, where TZD is the time zone offset. The W3 date and time format gives you 3 options for how to do the TZD: (Z or +hh:mm or -hh:mm)
This means that in javascript you can just use
const date = new Date().toISOString();
and it'll look like 2017-11-15T11:18:17.266Z, which is correct for a sitemap lastmod.

Managing multiple date formats in Javascript

Similar questions has been asked many times but I couldn't find a more concrete solution. The things I'm doing are:
I have a <input type="date"> inside my HTML which when clicked opens a calender with date in dd/mm/yyyy format.
I change the html5 date to timestamp to send to my db by Date.parse(html5Date) and in the server I modify the date and send it back to my Angular app.
I now convert the timestamp back to Date object by new Date(timestamp).To print the date in a human-friendly format inside a table I do [date.getDate(), date.getMonth() + 1, date.getFullYear()].join('/').
On edit (PUT request), I again capture the date from HTML, convert it to timestamp, send it to server and process the returning date back to html date.
Other than these, I also do a ton of functionalities like date comparison, adding hours to the dates, show time of the day etc inside the HTML:
Just these simple operations are over 120 lines of code which I think is ridiculous and error prone. I've looked into Angular Datepicker but it's a bit confusing. Also sometimes the HTML date is of type Object and sometimes it's String so Date.parse() gives error.
Are there any developer friendly methods that does : copy HTML5 date (from datepicker) --> change to timestamp (for angular&server) --> format timestamp back to string/object (for html)? Thank You :)
Note: Angular throws a lot of annoying error in console saying dateformat is wrong (being html date type) but doesn't stop code from running

Sounds like you are doing waaay to many conversions. I would argue that there should only be one way dates are represented: as Date objects in the programming language. There are only a few conversions that need to happen:
Date <=> Integer milliseconds since the epoch to pass to server
Date <=> String human-readable format to display to user
Any thing beyond this is asking for trouble. Comparisons can be made by casting to int date.getTime(), comparing, and casting back to Date. Ditto for additions. Note that Date.parse is implementation dependent in what it will accept, although all of them will accept ISO 8601 formatted date strings anything else is guesswork. Which means you will have to deal with converting strings by hand, something like the following:
var toDate = str => {
var splitter = str.indexOf("/") === -1 ? "-" : "/";
var [mon, day, year] = str.split(splitter);
return new Date(year, mon - 1, day);
};
var toDateString = date => {
return "" + date.getFullYear() + (date.getMonth() + 1) +...
};
Note that there's no validation, that's left as an exercise to the reader.
A WORD ABOUT MOMENT.JS
moment.js is awesome. Its also huge, its a kitchen-sink API with a heft to match. You're already loading angular, so think carefully before bulking the size of your payload with another huge library.

Moment.js is a powerful date formatting and manipulation library. A lot of things you can do in Moment.js are a single line of code, which makes life a lot easier. I agree, without using a library like this date formatting and handling can be a pain.
http://momentjs.com/
EDIT: fyi, I use this with my Angular app and find it extremely useful!

Get the given date format (the string specifying the format) in javascript or momentjs

Given a datestring, how can I get the format string describing that datestring?
Put another way, how can I get the format string that Date() or MomentJS (might be different for each, that's fine) would use to parse that datestring if one didn't pass an explicit format to use?
So given '2016-01-01' it should output something like 'YYYY-MM-DD', for example.
(I am aware this is a simple question and may have an answer somewhere, but it is difficult to word concisely, so I could only find questions and answers about how to parse datestrings or how to display dates. None about how to output the format itself.)

Consolidating information from Matt Johnson's answer, some comments, and my own contribution.
With Moment.js (version 2.10.7+), you can use the Creation Data API. Something like this in Node.js:
moment('2016-01-01 00:00:00').creationData().format
outputs
'YYYY-MM-DD HH:mm:ss'
Just as any date parsing is, there is ambiguity about the format of many datestrings due to things such as locale (the order of months and days are switched between the US and Europe, for example). But the above method is sufficient for me.

You can't, without having additional information, such as the locale. For example, 01/12/16 could be Jan 12, 2016, December 1, 2016, or December 16, 2001.
Even when you know the locale, there are several places in the real world where more than one date format is used, depending on context.
See https://en.wikipedia.org/wiki/Date_format_by_country
However, if you are just trying to determine which one of multiple known formats was used to parse the input string, moment has an API for that called Creation Data. For example:
var m = moment("2016/06/10", ["YYYY-MM-DD", "MM/DD/YYYY"], true);
var f = m.creationData().format; // "MM/DD/YYYY"

How do I get this JavaScript / moment code to work with both US and UK date formats?

How do I get this JavaScript / moment code to work with both US (MM/DD/YYYY) and UK (DD/MM/YYYY) date formats? Currently it only works with a UK format:
moment.fn.toJSON = function() {
return this.format("YYYY-MM-DDTHH:mmZ");
};
function submitform() {
document.getElementById("st_dt_tm").value =
moment(document.getElementById("inp-st").value, "DD/MM/YYYY HH:mm").toJSON();
return true;
};

There is no way, by date format alone, to know if it is DD/MM/YYYY or MM/DD/YYYY because of dates like 01/02/2016, which is a valid (but different) date either way.
Your choices are:
Use MM/DD/YYYY for US dates and DD-MM-YYYY for European dates, which is similar to what the JavaScript Date object does, but even that is not entirely useful -- more details here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/parse
The "best" option is to use some kind of cultural indicator flag -- either something passed in from the back-end code, or perhaps asking JavaScript to format a date for you and looking at what format they use -- and changing strings accordingly.

Manipulating dates in Javascript without the Date object

It appears I can't use the javascript Date object as it inherintly defaults to US dates when you initialise with a datestring. There is no way of passing any culture information to the date object
I.e. No matter what the clients locale settings are
var d = new Date("08/10/2009") will always create a date object representing the 10th August 2009 rather than the 8th October 2009 if the clients locale was the UK.
So given that my requirement is to be able to add/subtract days/months/years easily is there a clever way of doing this easily without the Date object
All i need to do is add a day to a date (or a string representation of a date). so if my code detects the locale setttings are in the US, when it sees a string like "10/08/2009" it whacks it up to "10/09/2009" but if it had detected it was in the UK it would have know it a uk string representation of a date and whacked it up to "09/10/2009"

For date manipulation and localization on JavaScript I always recommend the DateJS library.
This library abstracts the use of Date objects, has a very good localization options, powerful date parsing and formatting, and it also has a very nice fluent API.

If you know you are getting input formatted dd/mm/yyyy you can easily assemble the correct date.
function britDay(D){
D= D.match(/\d+/g);
return new Date(+D[2], D[1]-1, +D[0]);
}
toLocaleDateString will return the date in the format expected by the user.
Relying on the user input that obeys particular formatting rules is optimistic-
which is why most sites use separate, labeled inputs or select fields for the month, date and year.

You probably know that it's easy to add one day to a date, just add 86,400 * 1000 milliseconds to the date. It sounds like displaying in your locale is the issue; does Date.toLocaleString() not do the right thing for you?

dojo.date.locale.parse will be able to parse a formatted string according the locale of your choice. It has a table of cultural data based off unicode.org/cldr. See this article for more information.