What event marks when the christian calendar shifted from bc to ad?

you should go to google and ask/type: The Origin and History of the BCE/CE Dating System

the answer would be in this answer. someone already asked the question.  

swahili is an extremely rich and complex civilization, a set of ethnic groups articulated by a language and a series of cultural elements that fuse the african traditions, islam and indian culture. this growth was largely a result of the indian ocean trading network that brought in not just man-made riches, but also religious ideas, specifically islam, which had a significant impact on the local culture. furthermore, the development of the swahili language gave a common cultural unity to the people.