How to Initialize a JavaScript Date to Midnight

What is the best way to initialize a JavaScript Date to midnight?

The setHours method can take optional minutes, seconds and ms arguments, for example:

var d = new Date();
d.setHours(0,0,0,0);

That will set the time to 00:00:00.000 of your current timezone, if you want to work in UTC time, you can use the setUTCHours method.

Setting Javascript date object to midnight without being based on user computer date

Something like this I would guess: new Date(Date.UTC(year, month, day, hour, minute, second))

w3schools Reference

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

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');
```

Create a Date with a set timezone without using a string representation

using .setUTCHours() it would be possible to actually set dates in UTC-time, which would allow you to use UTC-times throughout the system.

~~You cannot set it using UTC in the constructor though, unless you specify a date-string.~~

Using new Date(Date.UTC(year, month, day, hour, minute, second)) you can create a Date-object from a specific UTC time.

Is the Javascript date object always one day off?

Notice that Eastern Daylight Time is -4 hours and that the hours on the date you're getting back are 20.

20h + 4h = 24h

which is midnight of 2011-09-24. The date was parsed in UTC (GMT) because you provided a date-only string without any time zone indicator. If you had given a date/time string w/o an indicator instead (new Date("2011-09-24T00:00:00")), it would have been parsed in your local timezone. (Historically there have been inconsistencies there, not least because the spec changed more than once, but modern browsers should be okay; or you can always include a timezone indicator.)

You're getting the right date, you just never specified the correct time zone.

If you need to access the date values, you can use getUTCDate() or any of the other getUTC*() functions:

var d,
  days;
d = new Date('2011-09-24');
days = ['Sun', 'Mon', 'Tues', 'Wed', 'Thurs', 'Fri', 'Sat'];
console.log(days[d.getUTCDay()]);