Complex sentence: Ten days after returning, on June 16, Kerry was promoted to the rank of Lieutenant, junior grade; on June 20, he left the "Gridley" for special Swift boat training at the Naval Amphibious Base in Coronado.
Split into simpler sentences: Ten days after returning, on June 16, Kerry was promoted to the rank of Lieutenant, junior grade. On June 20, he left the "Gridley" for special Swift boat training at the Naval Amphibious Base in Coronado.