That’s what I won’t understand – why in the movie “The End of the World” all of America celebrates “the last hours of the second millennium” on December 31, 1999?! The whole story of the film is bound to this! And nobody, nobody told them that they have a whole year ahead of the 21st century?! to
Indeed, human stupidity is inexplicable..."

Because December 31, 1999 was actually celebrated as the last day of the twentieth century :) And nobody really worried that the 21st century will actually start only in a year, the date is more interesting.