Conversation so far:
where is the dakota building in new york
The Dakota building is located on the northwest corner of 72nd Street and Central Park West in the Upper West Side of the borough of Manhattan in New York City
when was the dakota building in new york city erected
The Dakota building in New York City was completed in 1884.
who designed the dakota building in new york city
Henry Janeway Hardenbergh's architectural firm was commissioned to create the design for The Dakota building in New York City.
when did the dakota building become a new york city landmark
The Dakota building was designated a New York City Landmark in 1969

Question: how about a national landmark
Self-contained rewrite: when did the dakota building become a national landmark