Detecting an "invalid date" Date instance in JavaScript

Here's how I would do it:

if (Object.prototype.toString.call(d) === "[object Date]") {
  // it is a date
  if (isNaN(d.getTime())) {  // d.valueOf() could also work
    // date is not valid
  } else {
    // date is valid
  }
} else {
  // not a date
}

Update [2018-05-31]: If you are not concerned with Date objects from other JS contexts (external windows, frames, or iframes), this simpler form may be preferred:

function isValidDate(d) {
  return d instanceof Date && !isNaN(d);
}

Instead of using new Date() you should use:

var timestamp = Date.parse('foo');

if (isNaN(timestamp) == false) {
  var d = new Date(timestamp);
}

Date.parse() returns a timestamp, an integer representing the number of milliseconds since 01/Jan/1970. It will return NaN if it cannot parse the supplied date string.

You can check the validity of a Date object d via

d instanceof Date && isFinite(d)

To avoid cross-frame issues, one could replace the instanceof check with

Object.prototype.toString.call(d) === '[object Date]'

A call to getTime() as in Borgar's answer is unnecessary as isNaN() and isFinite() both implicitly convert to number.

My solution is for simply checking whether you get a valid date object:

Implementation

Date.prototype.isValid = function () {
    // An invalid date object returns NaN for getTime() and NaN is the only
    // object not strictly equal to itself.
    return this.getTime() === this.getTime();
};  

Usage

var d = new Date("lol");

console.log(d.isValid()); // false

d = new Date("2012/09/11");

console.log(d.isValid()); // true