CREATE TABLE table_27486 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( millions ) " TEXT, "Original U.S. air date" TEXT, "Prod. code" FLOAT)

How many different original U.S. air dates appear where the U.S. viewers were 2.5 million?
SELECT COUNT("Original U.S. air date") FROM table_27486 WHERE "U.S. viewers (millions)" = '2.5'
SELECT COUNT("Original U.S. air date") FROM "table_27486" WHERE "U.S. viewers (millions)" = '2.5'