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.