Complex sentence: He again returned to the Senate serving from 8 June 1971 until 1 September 1973 when he became the Managing Director of the International Monetary Fund, serving from 1 September 1973 until 18 June 1978.
Split into simpler sentences: He again returned to the Senate serving from 8 June 1971 until 1 September 1973. Afterwards he became the Managing Director of the International Monetary Fund, serving from 1 September 1973 until 18 June 1978.