CREATE TABLE table_name_15 (date VARCHAR, motogp_winner VARCHAR, grand_prix VARCHAR)

Name the date for motogp winner of casey stoner and grand prix of valencian grand prix
SELECT date FROM table_name_15 WHERE motogp_winner = "casey stoner" AND grand_prix = "valencian grand prix"
SELECT "date" FROM "table_name_15" WHERE "casey stoner" = "motogp_winner" AND "grand_prix" = "valencian grand prix"