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 } ```

Accepted Answer

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).

How does JS date comparison work?

Solution

Problem