user: Question: When was the date of appointment for St. Johnstone's manager? 
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Replaced by', 'Date of appointment']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Rangers', 'Walter Smith', 'Resigned', 'May 1998', 'Dick Advocaat', 'June 1998']
SQL:

assistant: SELECT "Date of appointment" FROM "Table" WHERE "Team" = 'St. Johnstone';