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Legit javascript UTC Date

I've been looking for a way to keep my dates in UTC in my JS application; however, in every case the Date's getTimezoneOffset() does not return a 0 which, I would imagine, should be the case -- and which would be seemingly important in casting dates between UTC and the local TZ.
See below examples of what I've tried, thanks in advance for any insight!
var TheDate = new Date( Date.UTC(2012, 10, 5) );
console.log(TheDate.getTimezoneOffset()) // => 300 (for me)
console.log(moment().utc().toDate().getTimezoneOffset()) // => 240 (for me)

ECAMScript Dates are inherently UTC and are just an offset from the ECMAScript epoch (a time value in milliseconds from 1970-01-01T00:00:00Z), nothing more. They have no associated timezone.
The host timezone offset is used:
When creating a Date to determine the equivalent UTC time and calculate the time value
In various methods that use local dates and times, such as getHours (vs getUTCHours)
The date itself does not know anything about timezones.
The value returned by getTimezoneOffset is based on the host system settings, the only relationship it has to the Date it's called on is that the offset is calculated for that Date. The method might be called getHostSystemTimezoneOffset, because that's what it returns.
In your code:
new Date(Date.UTC(2012, 10, 5))
creates a Date for 2012-11-05T00:00:00Z. Calling getTimezoneOffset on that date returns the host system offset for the equivalent local date and time, not the offset that was used in creating the Date.
There is no way to associate a timezone with a date without using a library (either one you write or one of the many existing libraries).
In your second example:
moment().utc()
just sets a flag to tell moment.js methods to use UTC for everything. Then using:
....toDate().getTimezoneOffset()
returns a Date object, then gets the host system timezone offset for that date as if you'd done:
new Date().getTimezoneOffset()
If you want to only use UTC, then use UTC methods for everything and ignore timezones completely (which I think is what you want to do).

Does Date.now() return different value for different timezones?

The definition for Date.now() is not clear for me. As per definition "The Date. now() is an inbuilt function in JavaScript which returns the number of milliseconds elapsed since January 1, 1970, 00:00:00 UTC.". So, does it mean that it will give same value for Date.now() in all timezone?
The current date and time, picked for calculation, is my local timezone or UTC ?
I have same query for java.util.Date getTime() method.

Yes, Date.now() will give you the same UTC timestamp independent of your current timezone. Such a timestamp, rather a point in time, does not depend on timezones.
The Java equivalent new Date() gives you the exact same thing.
Check out Coordinated Universal Time (UTC) for more information.
FYI: Don't use new Date() in Java as it's a legacy class. Use Instant.now() that is from the new java.time API that is much more robust and has a nicer design.

Set date() to midnight in users timezone with moment.js

I use moment.js to display a UTC date in the users local timezone:
var date = new Date(Date.UTC(2016,03,30,0,0,0));
var now = new Date();
var diff = (date.getTime()/1000) - (now.getTime()/1000);
var textnode = document.createTextNode(moment(date).format('dddd, DD.MM.YYYY') + ' a las ' + moment(date).format('HH:mm A'));
document.getElementsByClassName("date")[0].appendChild(textnode.cloneNode(true));
I later use the diff variable to show a countdown timer.
I would like to show a different countdown timer to everyone in their local time zone. (Using the difference till its midnight in their time zone, not in UTC)
But I am struggeling to get it work. Instead of using var date = new Date(Date.UTC(2016,03,30,0,0,0)); I probably need to use some function of moment.js that gives me till midnight in the users time zone.
The best example would be new years eve. If I use UTC everyone would have the same counter (9 hours left) but in different parts of the world this wouldn't make sense. For someone in australia it should be 2 hours left, and for someone in the US 14 hours.

I'm not sure that I fully understand your question, but I'll give you some general advice and tips.
When using moment.js, there is very little need to ever use the Date object. Only use it for interacting with other APIs that expect a Date object.
To get a moment in UTC, just use moment.utc(...), passing the appropriate arguments, such as moment.utc([2016,3,30]) or moment.utc('2016-04-30') for midnight April 30th UTC.
If you want to convert that back to the user's local time, use the .local() function. For example, moment.utc('2016-04-30').local() will create a moment with the equivalent local time to the UTC time provided.
If you want a moment in the user's local time, then that would be moment(...), such as moment([2016,3,30]) or moment('2016-04-30') for midnight April 30th local time.
You can difference two moments using the diff function, which can give the answer in specific units, such as m1.diff(m2, 'seconds') where m1 and m2 are moment objects.
You don't need to call format twice. Just encapsulate any text you want outputed with square brackets. .format('dddd, DD.MM.YYYY [a las] HH:mm A')
You might look into moment's locale support. If I'm not mistaken, "a las" indicates Spanish, however it's not always "a las", but sometimes "a la", if the hour is 1. Also, moment only uses those words in its .calendar() function, such as when producing a phrase like "mañana a las 13:17". A regular date formatted with .format('LLLL') in the Spanish locale would be something like: "sábado, 19 de marzo de 2016 13:17". So, you might want to verify that "a las" is exactly what you want in every case.
The title to this question was how to set a date to midnight. For that, I recommend using moment's startOf function. m.startOf('day') will give set the moment m to the start of the day, which is usually midnight. Keep in mind that not every local day actually starts at midnight in every time zone. Due to anomalies like daylight saving time, some days might start at 1:00. For example, this occurs in Brazil on October 16th this year.
Also, if you created the moment in UTC mode, you may wish to convert it back to local mode first before setting it to the start of the day. If you don't want to change the original moment object, be sure to clone it first.
Putting this all together:
var m1 = moment.utc([2016,3,30]);
var m2 = m1.clone().local().startOf('day');
var now = moment();
var diff = m1.diff(now, 'seconds');

How to create Javascript Date() object given Country ISO code?

Consider I'm from India and my Country ISO code is IN or IND or 356 (http://en.wikipedia.org/wiki/ISO_3166-1). How could I generate a JavaScript Date object with that? That is, in IST.
In Python, we have astimezone(). Anything similar to that is available in JavaScript?
PS: jQuery or any trusted third party API is allowed.

JavaScript in the browser is not a timezone aware language. It has the ability to work with only two timezones: UTC and local timezone where local timezone depends on the timezone of the Browser or the Operating System that the Browser runs on.
If your browser (and your user's browser) is set to IST, then all dates will display in IST. If your browser is not set to IST, then you're sort of out of luck.
Now, you can do some tricks. For example, you can do the following:
var d = new Date(); // creates a date in user's local timezone
var istD = new Date(d.getTime() + (d.getTimezoneOffset() + 330) * 60000)
// 330 is the IST offset (5h 30m == 330m)
istD is now a date object that will "print the date in IST". Note that this date object is still in the user's local timezone, but as long as you don't display the timezone part, it will appear to be in IST.
There is a small problem with Daylight Saving Time. Since IST does not have DST, this problem is minimized, but still exists if the user's local timezone has DST and you are just a few hours off from the DST offset. You might be able to play around with the Date.getUTC* functions to circumvent the DST issues, however always remember that there is no way to correctly do timezones in clientside JavaScript. There is no library in existence today that correctly handles all TimeZone rules, including Daylight Savings rules for all historical dates.

You can set the date with UTC and then you can convert it into LocaleString. You can see Date.prototype.toLocaleString() for more refrence.
And you can also see the question asked on stack overflowHow do I display a date/time in the user's locale format and time offset?.
Thanks

There is no directly API.But you can coding it by your self.
<html>
<body onload="getTimeByTimeZone(5.5)">
<script type="text/javascript">
function getTimeByTimeZone(offset){
var d = new Date()
localTime = d.getTime();
localOffset=d.getTimezoneOffset()*60000;
utcTime=localTime + localOffset;//get utc time
st=utcTime+3600000*offset;//get the specified timezone time
stime=new Date(st);
document.write("The specified time is :"+stime.toString());
}
</script>
</body>
</html>
Hope this code is helpful to you.

Set JavaScript countdown to end at midnight PST?

I am trying to get a countdown to end Monday # midnight PST. I thought I had it working a week ago but apparently not.
I am using date.js
var monday = Date.today().next().monday();
var UTCmonday = new Date(monday.getTime() + monday.getTimezoneOffset() * 60000);
var PSTmonday = new Date(UTCmonday.setHours(UTCmonday.getHours() + 9));
$('#defaultCountdown').countdown({until: UTCmonday});
I think the problem is in determining UTC time? Am I right? How do I work this out?

Assuming you Pacific Standard Time, then you need to remember that PST === UTC-8
Thus, your third line would be
var PSTmonday = new Date(UTCmonday.setHours(UTCmonday.getHours() - 8));
The problem with this is that this will fail if the UTC is any earlier than 8am, since you can't pass a negative number into setHours.
Since you're using Datejs, why not use its full capabilities for changing the timezone?
http://code.google.com/p/datejs/wiki/APIDocumentation#setTimezone

Getting the time strictly in PST doesn't make much sense, as for almost half of the year PST isn't observed in the Pacific time zone. PST (UTC-8) is observed in the winter, and PDT (UTC-7) is observed in the summer. You can't represent Pacific Time as just a fixed offset, and unless it happens to be your own local time zone, you can't easily determine the transition between them without a time zone database. See the timezone tag wiki.
Also, date.js has been abandoned. I can't recommend any solution that continues its use. Support for the setTimezone method that Dancrumb suggested is culture specific, and it still doesn't take daylight saving time into consideration.
Instead, I recommend trying moment.js. You can use the moment-timezone add-on to work with the America/Los_Angeles zone - which is a good exemplar of US Pacific time. Make sure your moment-timezone-data.js file includes at least this zone. Then you can do the following:
var m = moment().tz('America/Los_Angeles').day(7).startOf('day');
var s = m.toISOString(); // if you need a string output representing UTC
var dt = m.toDate(); // if you need an actual JavaScript Date object
Let's break that down a bit:
moment() gets you the current time
tz('America/Los_Angeles') adjusts the time to the timezone you are interested in.
day(7) advances the time to the next Monday.
startOf('day') snaps the time back to midnight.