user: Question: The length of 30 minutes aired on what date?
Columns: ['Title', 'Length', 'Writer', 'Director', 'Airdate']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['"Episode 1"', '60 minutes', 'Sarah Phelps', 'Coky Giedroyc', '18 December 2007']
SQL:

assistant: SELECT "Airdate" FROM "Table" WHERE "Length" = '30 minutes';