CREATE TABLE table_75766 ("Date" TEXT, "Tournament" TEXT, "Winning score" TEXT, "Margin of victory" TEXT, "Runner ( s ) -up" TEXT)

Which Date has a Runner(s)-up of bernhard langer, and a Tournament of at&t championship?
SELECT "Date" FROM table_75766 WHERE "Runner(s)-up" = 'bernhard langer' AND "Tournament" = 'at&t championship'
SELECT "Date" FROM "table_75766" WHERE "Runner(s)-up" = 'bernhard langer' AND "Tournament" = 'at&t championship'