I expanded just slightly on the isValidDate function Thorbin posted above (using a regex). We use a regex to check the format (to prevent us from getting another format which would be valid for Date). After this loose check we then actually run it through the Date constructor and return true or false if it is valid within this format. If it is not a valid date we will get false from this function.
function isValidDate(dateString) {
var regEx = /^\d{4}-\d{2}-\d{2}$/;
if(!dateString.match(regEx)) return false; // Invalid format
var d = new Date(dateString);
var dNum = d.getTime();
if(!dNum && dNum !== 0) return false; // NaN value, Invalid date
return d.toISOString().slice(0,10) === dateString;
}
/* Example Uses */
console.log(isValidDate("0000-00-00")); // false
console.log(isValidDate("2015-01-40")); // false
console.log(isValidDate("2016-11-25")); // true
console.log(isValidDate("1970-01-01")); // true = epoch
console.log(isValidDate("2016-02-29")); // true = leap day
console.log(isValidDate("2013-02-29")); // false = not leap day