WITH t AS
(
SELECT '2011.01.01' dt, 'A' gb FROM dual
UNION ALL SELECT '2011.04.02', 'A' FROM dual
UNION ALL SELECT '2012.01.01', 'B' FROM dual
UNION ALL SELECT '2012.07.01', 'C' FROM dual
UNION ALL SELECT '2012.10.01', 'C' FROM dual
UNION ALL SELECT '2012.11.01', 'C' FROM dual
UNION ALL SELECT '2012.12.01', 'B' FROM dual
UNION ALL SELECT '2013.02.01', 'A' FROM dual
UNION ALL SELECT '2014.02.01', 'A' FROM dual
)
SELECT MAX(dt)
  FROM (SELECT dt, gb
             , DECODE(LAG(gb) OVER(ORDER BY dt), gb, 0, 1) flag
          FROM t
        )
 WHERE flag = 1
;