Sergeant Zimmer was a member of Battery A, 57th Field Artillery Battalion, 7th Infantry Division. He was listed as Missing in Action while fighting the enemy along the eastern shore of the Chosin Reservoir in North Korea on December 6, 1950. He was presumed dead on December 31, 1953. His remains were not recovered. In August and September 2004, an joint U.S. and Korean People's Army recovery team conducted a Joint Recovery Operation in the vicinity of the East Chosin Reservoir, where Zimmer went missing. The team recovered possible human remains and sent them to the DPAA laboratory for analysis. Laboratory analysis and circumstantial evidence were used in the identification of his remains. His name permanently inscribed on the Courts of the Missing at the Honolulu Memorial.