let unix_timestamp = 1549312452;

// Create a new JavaScript Date object based on the timestamp
// multiplied by 1000 so that the argument is in milliseconds, not seconds
var date = new Date(unix_timestamp * 1000);

// Hours part from the timestamp
var hours = date.getHours();

// Minutes part from the timestamp
var minutes = "0" + date.getMinutes();

// Seconds part from the timestamp
var seconds = "0" + date.getSeconds();

// Will display time in 10:30:23 format
var formattedTime = hours + ':' + minutes.substr(-2) + ':' + seconds.substr(-2);

console.log(formattedTime);

For more information regarding the Date object, please refer to MDN or the ECMAScript 5 specification.

Just create a Date object from it and do .getTime() or use Date.parse():

var d = new Date("Wed Jun 20 19:20:44 +0000 2012");
d.getTime(); //returns 1340220044000

//OR

Date.parse("Wed Jun 20 19:20:44 +0000 2012"); //returns 1340220044000

Works great if your "human time" string is in a format that the Date constructor understands (which the example you posted is).

EDIT

Realized you may mean a Unix timestamp, which is seconds passed since the epoch (not ms like JS timestamps). In that case simply divide the JS timestamp by 1000:

//if you want to truncate ms instead of rounding just use Math.floor()
Math.round(Date.parse("Wed Jun 20 19:20:44 +0000 2012") / 1000); //returns 1340220044

Split the string into its parts and provide them directly to the Date constructor:

Update:

var myDate = "26-02-2012";
myDate = myDate.split("-");
var newDate = new Date( myDate[2], myDate[1] - 1, myDate[0]);
console.log(newDate.getTime());

Updated: Also, you can use a regular expression to split the string, for example:

const dtStr = "26/02/2012";
const [d, m, y] = dtStr.split(/-|\//); // splits "26-02-2012" or "26/02/2012"
const date = new Date(y, m - 1, d);
console.log(date.getTime());

The value 1382086394000 is probably a time value, which is the number of milliseconds since 1970-01-01T00:00:00Z. You can use it to create an ECMAScript Date object using the Date constructor:

var d = new Date(1382086394000);

How you convert that into something readable is up to you. Simply sending it to output should call the internal (and entirely implementation dependent) toString method* that usually prints the equivalent system time in a human readable form, e.g.

Fri Oct 18 2013 18:53:14 GMT+1000 (EST) 

In ES5 there are some other built-in formatting options:

• toDateString
• toTimeString
• toLocaleString

and so on. Note that most are implementation dependent and will be different in different browsers. If you want the same format across all browsers, you'll need to format the date yourself, e.g.:

alert(d.getDate() + '/' + (d.getMonth()+1) + '/' + d.getFullYear());

* The format of Date.prototype.toString has been standardised in ECMAScript 2018. It might be a while before it's ubiquitous across all implementations, but at least the more common browsers support it now.