The 1989 movie takes place in an alternate 1979, based on the American Express Card advertisement reference and dates on props. The sequel, Batman Returns can take place no further back than 1983, based on the Love Connection reference and Penguin's birth certificate prop. It's interesting that both dates are pretty much exactly a decade prior to the real-world release years. The prop people/script supervisor/director must have been proceeding on agreement to this rounded off timeline.