Complex sentence: He was Canada's second longest-serving Prime Minister, with two separate terms as Prime Minister (1867--1873,1978--1891), for a total of 18 years and 359 days.
Split into simpler sentences: He was Canada's second longest-serving Prime Minister, with two separate terms as Prime Minister (1867--1873,1878--1891). MacDonald was the second-longest serving Prime Minister, in office for a total of 18 years, 359 days.