user: Question: When the face value is 42¢, what was the issue's date?
Columns: ['Ecosystem', 'Date of issue', 'Place of issue', 'No. stamps in sheet', 'Face Value', 'Printer']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Sonoran Desert', 'April 6, 1999', 'Tucson, Arizona', 10.0, '33¢', 'Banknote Corporation of America']
SQL:

assistant: SELECT "Date of issue" FROM "Table" WHERE "Face Value" = '42¢';