user: Question: On which date was the class 157?
Columns: ['Class', 'Date', 'Builder', 'No. built', '1919 nos.', 'LMS Class', 'LMS nos.']
Types: ['real', 'text', 'text', 'real', 'text', 'text', 'text']
Sample row: [157.0, '1879-81', 'G&SWR Kilmarnock', 12.0, '720-5', '1P', '14001-2']
SQL:

assistant: SELECT "Date" FROM "Table" WHERE "Class" = 157.0;