Passage: In 2005, Batman Begins was released by Warner Bros. as a reboot of the film series; directed by Christopher Nolan and starring Christian Bale as Batman. Its sequel, The Dark Knight (2008), set the record for the highest grossing opening weekend of all time in the U.S., earning approximately $158 million, and became the fastest film to reach the $400 million mark in the history of American cinema (eighteenth day of release). These record-breaking attendances saw The Dark Knight end its run as the second-highest domestic grossing film (at the time) with $533 million, bested then only by Titanic. The film also won two Academy Awards, including Best Supporting Actor for the late Heath Ledger. It was eventually followed by The Dark Knight Rises (2012), which served as a conclusion to Nolan's film series.

Question: How long did it take for The Dark Knight to reach sales of $400 million?
eighteenth day of release