_checkFormat example

if (!is_string($value) && !is_int($value) && !is_float($value) &&
            !is_array($value) && !($value instanceof Zend_Date)) {
            $this->_error(self::INVALID);
            return false;
        }

        $this->_setValue($value);

        if (($this->_format !== null) || ($this->_locale !== null) || is_array($value) ||
             $value instanceof Zend_Date) {
            if (!Zend_Date::isDate($value, $this->_format, $this->_locale)) {
                if ($this->_checkFormat($value) === false) {
                    $this->_error(self::FALSEFORMAT);
                } else {
                    $this->_error(self::INVALID_DATE);
                }
                return false;
            }
        } else {
            if (!preg_match('/^\d{4}-\d{2}-\d{2}$/', $value)) {
                $this->_format = 'yyyy-MM-dd';
                $this->_error(self::FALSEFORMAT);
                $this->_format = null;