Catholicism has always been a huge religion, and has been in control of the world for previous ages. For that reason, they were the ones who could dictate when to start the timeline, and chose the birth of Christ as that starting point.
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A primary source tells you about a certain event that happened in the past and it's from that event. When studying historical events, historians want their information to be precise and correct. primary sources are firsthand, contemporary accounts of a certain event that was recorded by somebody who witnessed or experienced that event. Primary sources includes the original stuff and it's very important.