Answer by Dex for Detecting an "invalid date" Date instance in JavaScript

None of these answers worked for me (tested in Safari 6.0) when trying to validate a date such as 2/31/2012, however, they work fine when trying any date greater than 31.

So I had to brute force a little. Assuming the date is in the format mm/dd/yyyy. I am using @broox answer:

Date.prototype.valid = function() {
    return isFinite(this);
}    

function validStringDate(value){
    var d = new Date(value);
    return d.valid() && value.split('/')[0] == (d.getMonth()+1);
}

validStringDate("2/29/2012"); // true (leap year)
validStringDate("2/29/2013"); // false
validStringDate("2/30/2012"); // false