SELECT dttm_s
     , dttm_e
     , TIMESTAMPDIFF(second, dttm_s, dttm_e) sec
  FROM (SELECT dt
             , tm
             , CONCAT(dt, ' ', tm) dttm_s
             , (SELECT CONCAT(dt, ' ', tm)
                  FROM t s
                 WHERE msg = 'state up'
                   AND s.dt >= m.dt
                   AND ( (s.dt = m.dt AND s.tm > m.tm) OR (s.dt > m.dt) )
                 ORDER BY s.dt, s.tm
                 LIMIT 1
                ) dttm_e
          FROM t m
         WHERE msg = 'state down'
        ) a
;

설마 했었는데.

역시 마농님께서 답변 주셨네요.

감사합니다.