user: Question: Harry L Stephen rides a Norton machine on what date?
Columns: ['Rider', 'Date', 'Place', 'Race', 'Event', 'Machine']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Victor Surridge', '27 June 1911', 'Glen Helen', '1911 Isle of Man TT', 'Practice', 'Rudge-Whitworth']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Machine" = 'Norton' AND "Rider" = 'Harry L Stephen';