How to clone a Date object?

This is the cleanest approach

let date = new Date() 
let copyOfDate = new Date(date.valueOf())

console.log(date);
console.log(copyOfDate);

Update for 2021:

In one respect the notion of cloning a Date object sounds grander than it really is. As far as I can tell, there’s only one piece of instance data, and that is the stored time. What we’re really doing is making a new object with the same time.

Whatever may have been the case in the past, the new Date() constructor definitely accepts a Date object as a single argument:

var date = new Date();      //  with or without an argument
var date2 = new Date(date); //  clone original date

The documentation at https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-date, step 4(b) indicates that a Date object is definitely acceptable, and that this is equivalent to new Date(date.valueOf()), as suggested by some of the answers above. As I said, all you’re really doing is making a new Date object with the same time as the other.

You’ll also find that the documentation at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date has been updated to include this.

Use the Date object's getTime() method, which returns the number of milliseconds since 1 January 1970 00:00:00 UTC (epoch time):

var date = new Date();
var copiedDate = new Date(date.getTime());

In Safari 4, you can also write:

var date = new Date();
var copiedDate = new Date(date);

...but I'm not sure whether this works in other browsers. (It seems to work in IE8).

var orig = new Date();
var copy = new Date(+orig);

console.log(orig, copy);