user: Question: On how many different dates was the race at the Silverstone circuit?
Columns: ['Round', 'Circuit', 'Date', 'Pole Position', 'Fastest Lap', 'Winning Drivers', 'Winning Team']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Oulton Park', 'April 11', 'Daniel Brown Stuart Hall', 'Hector Lester Alan Simonsen', 'Daniel Brown Stuart Hall', 'Rollcentre Racing']
SQL:

assistant: SELECT COUNT("Date") FROM "Table" WHERE "Circuit" = 'Silverstone';