Complex sentence: On August 5 1861 Williston was commissioned in the 2nd United States Artillery as a Second Lieutenant in San Francisco, California, later that year, on September 27, he was promoted to First Lieutenant.
Split into simpler sentences: On August 5,1861, Williston was commissioned in the 2nd United States Artillery as a second lieutenant in San Francisco, California. Later that year, on September 27, he was promoted to first lieutenant.