Conversation so far:
Where did Tom Lehrer go to school?
Tom Lehrer was considered a child prodigy and entered Harvard College at the age of 15 after graduating from Loomis Chaffee School.
Did Tom Lehrer go to graduate school?
Tom Lehrer attended graduate school for mathematics at Harvard University.
Did Tom Lehrer ever join the military?
Tom Lehrer was drafted into the U.S. Army from 1955 to 1957, working at the NSA.
What was Tom Lehrer's rank in the military?
Tom Lehrer served as an enlisted soldier, achieving the rank of Specialist Third Class.

Question: When did he finish his military career?
Self-contained rewrite: When did Tom Lehrer finish his military career?