Complex sentence: During this time, he had spells as temporary commander of the brigade while Johnston was on leave and also had a month long period from December 1916 to January 1917 in command of 3rd New Zealand (Rifle) Brigade.
Split into simpler sentences: During this time, he had spells as temporary commander of the brigade while Johnston was on leave. He also had a month long period from December 1916 to January 1917 in command of 3rd New Zealand (Rifle) Brigade.