What does the plus sign do in '+new Date'

Question

that's the + unary operator, it's equivalent to:

function(){ return Number(new Date); }

see: http://xkr.us/articles/javascript/unary-add/

and in MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators#Unary_plus

JavaScript is loosely typed, so it performs type coercion/conversion in certain circumstances:

http://blog.jeremymartin.name/2008/03/understanding-loose-typing-in.html
http://www.jibbering.com/faq/faq_notes/type_convert.html

Other examples:

>>> +new Date()
1224589625406
>>> +"3"
3
>>> +true
1
>>> 3 == "3"
true

Here is the specification regarding the "unary add" operator. Hope it helps...

A JavaScript date can be written as a string:

Thu Sep 10 2015 12:02:54 GMT+0530 (IST)

or as a number:

1441866774938

Dates written as numbers, specifies the number of milliseconds since January 1, 1970, 00:00:00.

Coming to your question it seams that by adding '+' after assignment operator '=' , converting Date to equal number value.

same can be achieve using Number() function, like Number(new Date());

var date = +new Date(); //same as 'var date =number(new Date());'

Just to give some more info:

If you remember, When you want to find the time difference between two Date's, you simply do as following;

var d1 = new Date("2000/01/01 00:00:00"); 
var d2 = new Date("2000/01/01 00:00:01");  //one second later

var t = d2 - d1; //will be 1000 (msec) = 1 sec

typeof t; // "number"

now if you check type of d1-0, it is also a number:

t = new Date() - 0; //numeric value of Date: number of msec's since 1 Jan 1970.
typeof t; // "number"

that + will also convert the Date to Number:

typeof (+new Date()) //"number"

But note that 0 + new Date() will not be treated similarly! it will be concatenated as string:

0 + new Date() // "0Tue Oct 16 05:03:24 PDT 2018"