Complex sentence: The house was added to the U.S. National Register of Historic Places on June 19,1972, exactly 24 years later, in 1996 it was declared a National Historic Landmark by the United States Department of the Interior.
Split into simpler sentences: The house was added to the U.S. National Register of Historic Places on June 19,1972. Exactly 24 years later, in 1996, it was declared a National Historic Landmark by the United States Department of the Interior.