user: Question: On what air date were there 2.15 million u.s. viewers?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [131.0, 1.0, '"4:30 AM (Apparently They Were Traveling Abroad)"', 'Clark Mathis', 'Mark Schwahn', 'September14,2009', '3X5301', '2.55']
SQL:

assistant: SELECT "Original air date" FROM "Table" WHERE "U.S. viewers (millions)" = '2.15';