CREATE TABLE table_22380270_1 (original_air_date VARCHAR, us_viewers__millions_ VARCHAR)

On what air date were there 2.15 million u.s. viewers?
SELECT original_air_date FROM table_22380270_1 WHERE us_viewers__millions_ = "2.15"
SELECT "original_air_date" FROM "table_22380270_1" WHERE "2.15" = "us_viewers__millions_"