user: Question: Name the date for motogp winner of Casey Stoner and grand prix of Valencian Grand Prix
Columns: ['Round', 'Date', 'Grand Prix', 'Circuit', '125cc winner', '250cc winner', 'MotoGP winner', 'Report']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, '9 March', 'Qatar Grand Prix ‡', 'Losail', 'Sergio Gadea', 'Mattia Pasini', 'Casey Stoner', 'Report']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "MotoGP winner" = 'Casey Stoner' AND "Grand Prix" = 'Valencian Grand Prix';