user: Question: When has Clark University joined the Conference?
Columns: ['Institution', 'Location', 'Nickname', 'Founded', 'Type', 'Enrollment', 'Joined']
Types: ['text', 'text', 'text', 'real', 'text', 'real', 'real']
Sample row: ['Babson College', 'Wellesley, Massachusetts', 'Beavers', 1919.0, 'Private/Non-sectarian', 3200.0, 1985.0]
SQL:

assistant: SELECT "Joined" FROM "Table" WHERE "Institution" = 'Clark University';