Conversation so far:
When did Abner Doubleday have a relationship with baseball?
Abner Doubleday is more widely known as the supposed inventor of the game of baseball, in Elihu Phinney's cow pasture in Cooperstown, New York, in 1839.
How did Abner Doubleday invent baseball?
A letter states Abner Doubleday invented baseball as a modified version of town ball, with four bases on the field and batters who attempted to hit tosses from a pitcher.

Question: When did he invent baseball?
Self-contained rewrite: When did Abner Doubleday invent baseball?