function filter_validate_date($format) {
  return function($date) {
    $dt = DateTime::createFromFormat($format, $date);
    return $dt && $dt->format($format, $date);
  }
}
var filter_validate_date_ymd = filter_validate_date('Y-m-d');

function isValidDateYmd($date) {
  return filter_var($date, FILTER_CALLBACK, array('options' => 'filter_validate_date_ymd'));
}