user: Question: What is the date of the circuit of Nürburgring, which had Graham Hill as the winning driver?
Columns: ['Race', 'Circuit', 'Date', 'Pole position', 'Fastest lap', 'Winning driver', 'Constructor', 'Tyre', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Dutch Grand Prix', 'Zandvoort', '20 May', 'John Surtees', 'Bruce McLaren', 'Graham Hill', 'BRM', 'D', 'Report']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Winning driver" = 'Graham Hill' AND "Circuit" = 'Nürburgring';