Ottumwa IA is 15 hours behind South Korea. When Colonel Potter called Radar, Mrs. O'Reilly informed him that Radar had taken an evening job in town. Although since no time was given the unit was out at the O club at night. Even though it was evening in Iowa it would have been very late the previous night in South Korea or possibly very early in the morning when they called.
Former 4077th M*A*S*H clerk Corporal Walter "Radar" O'Reilly (Gary Burghoff) sends a letter from his home of Ottumwa, Iowa. After hearing of his post-army success as a farmer, Major Winchester (David Ogden Stiers), quips, "Today Ottumwa, tomorrow Keokuk". Both are cities in southeastern Iowa. At the time of the Korean War, Ottumwa had a population of 33,000 and Keokuk's population was 16,000.
Colonel Potter says that Radar is about 10,000 miles away. Actually, it's about 6,400 miles between South Korea and Ottumwa, IA.