user: Question: What is Date, when Outcome is "Winner", and when Surface is "Grass"?
Columns: ['Outcome', 'Date', 'Tournament', 'Surface', 'Opponent in the final', 'Score in the final']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Winner', '17 September 1989', 'Geneva , Switzerland', 'Clay', 'Guillermo Pérez Roldán', '6–4, 7–5']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Outcome" = 'Winner' AND "Surface" = 'Grass';