Complex sentence: It was declared a National Historic Landmark in 1960, was listed on the National Register of Historic Places in 1966, and since 1984, has been owned and operated by the National Trust for Historic Preservation.
Split into simpler sentences: It was declared a National Historic Landmark in 1960, was listed on the National Register of Historic Places in 1966. Since 1984, it has been owned and operated by the National Trust for Historic Preservation.