user: Question: what is all the  broadcast date when viewers were 8.4 millions
Columns: ['Episode', 'Broadcast date', 'Run time', 'Viewers (in millions)', 'Archive']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['" Episode 1 "', '3February1968', '24:53', '7.2', '16mm t/r']
SQL:

assistant: SELECT "Broadcast date" FROM "Table" WHERE "Viewers (in millions)" = '8.4';