To the extent that Diamond gives an answer to this question, he does so in Chatper 19. However, he does not give an exact date.
The Bantu domination of sub-Saharan Africa is not recorded in the historical record. Therefore, its dates must be inferred from other evidence. Diamond cites archaeological and linguistic evidence to support his estimates. He says (p. 394 in the paperback edition of the book) that the Bantu expansion out of their West African homeland “may have begun as early as 3000 B.C.” He then goes on to say that “By soon after 1000 B.C.” Bantu peoples had arrived in the Great Rift Valley area of Eastern Africa. By 500 A.D., they had completed their expansion.
We do not know an exact date for this expansion and domination, so we can only give approximate dates.