case 'day':
            $letterCriteria->add(new \Criteria('letter_created', strtotime(date('Y/m/d')), '>='));
            $letterCriteria->add(new \Criteria('letter_created', strtotime(date('Y/m/d')) + 86400, '<='));
            $letterCriteria->setSort('letter_created');
            $letterCriteria->setOrder('ASC');