public function setTime($value = null)
    {
        switch ($value) {
            case 'today':
                $this->_params['time'] = 'today';
                break;
            case 'this_week':
                $this->_params['time'] = 'this_week';
                break;
            case 'this_month':
                $this->_params['time'] = 'this_month';
                break;
            case 'all_time':
                $this->_params['time'] = 'all_time';
                break;
            case null:
                unset($this->_params['time']);
            default:
                require_once 'Zend/Gdata/App/InvalidArgumentException.php';
                throw new Zend_Gdata_App_InvalidArgumentException(
                    'Unknown time value');
                break;
        }
        return $this;
    }