How does JS date comparison work?

Question

Let's assume I have a proper Date object constructed from the string: "Tue Jan 12 21:33:28 +0000 2010" .

var dateString = "Tue Jan 12 21:33:28 +0000 2010";
var twitterDate = new Date(dateString);

Then I use the < and > less than and greater than comparison operators to see if it's more or less recent than a similarly constructed Date . Is the algorithm for comparing dates using those operators specified, or is it specifically unspecified , like localeCompare ? In other words, am I guaranteed to get a more recent date, this way?

var now = new Date();
if (now < twitterDate) {
    // the date is in the future
}

Answer 1

Relational operations on objects in ECMAScript rely on the internal ToPrimitive function (with hint number) that you can access, when it is defined, using valueOf .

Try

var val = new Date().valueOf();

You'll get the internal value of the date which is, as in many languages, the number of milliseconds since midnight Jan 1, 1970 UTC (the same that you would get using getTime() ).

This means that you're, by design, ensured to always have the date comparison correctly working.

This article will give you more details about toPrimitive (but nothing relative to comparison).

Answer 2

I think yes. Using if (now < twitterDate) , it evaluates to if (now.valueOf()<twitterDate.valueOf()) . valueOf() delivers the number of milliseconds passed since 01/01/1970 00:00:00, so the comparison of those 2 numbers is valid.

check it like this

var then = new Date("Tue Jan 12 21:33:28 +0000 2010")
   ,now  = new Date;

console.log(then.valueOf(),'::',now.valueOf(),'::',now<then);
  //=> 1263332008000 :: 1352365105901 :: false

Answer 3

Date values in Javascript are numbers, as stated in the ECMA Script specification . So the Date values are compared as numbers.

This is a demo of your code (I set twitterDate in the future).

(function(){
    var dateString = "Tue Jan 12 21:33:28 +0000 2014";
    var twitterDate = new Date(dateString);

    var now = new Date();
    if (now < twitterDate) {
         document.write('twitterDate is in the future');
    }
    else
    {
        document.write('twitterDate is NOT in the future');
    }

})()​