Complex sentence: Its building, originally to be called the Durant Building, was designed and began construction in 1919 when Durant was president, was completed in 1923 (Sloan became president that year) and officially dedicated as the General Motors Building in 1929.
Split into simpler sentences: Its building, originally to be called the Durant Building, was designed and began construction in 1919 when Durant was president, was completed in 1923. Alfred P. Sloan became president that year and officially dedicated as the General Motors Building in 1929.