Conversation so far:
Who was in a business partnership with John D. Rockefeller?
In 1859, John D. Rockefeller went into business with a partner, Maurice B. Clark.
What kind of business were John D. Rockefeller and Maurice B. Clark into?
John D. Rockefeller and Maurice Clark went into the produce commission business that transitioned oil refineries.
What other business partnership besides Clark's did John D. Rockefeller have?
In 1866, William Rockefeller Jr., John D. Rockefeller's brother, built another oil refinery in Cleveland and brought John into the partnership.
What else can you tell me about John D. Rockefeller that is interesting besides his partnerships?
The firm established by John D. Rockefeller, Andrews & Flagler was the predecessor of the Standard Oil Company.
What did John D. Rockefeller's Rockefeller, Andrews and Flagler firm do?
John D. Rockefeller's firm refined petroleum and marketed by-products of the refining process.

Question: when did they buy out the clark brothers?
Self-contained rewrite: When did John D. Rockefeller buy out the Clark brothers?