Detecting an "invalid date" Date instance in JavaScript

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.

Here's how I would do it:

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

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);
}

Update [2021-02-01]: Please note that there is a fundamental difference between "invalid dates" (2013-13-32) and "invalid date objects" (new Date('foo')). This answer does not deal with validating date input, only if a Date instance is valid.