Although time does not have a beginning and an end, when it comes to the study of history it is necessary to define an anchor point on the timeline as a starting point for the counting of years. The discussion of calendars in Lecture 7 it will show that different civilizations use different ways to divide time into eras.
Many civilizations used more than one starting point, for example the first days of the reigns of successive emperors, which leads to the adoption of several eras and identification of dates by only positive numbers ("the fifth year of the reign of Emperor Tai Zong of the Tang dynasty"). The South American calendar was based on a zero point so far back in time (at 3113 BC) that for all practical applications the identification of dates also invoked only positive numbers.
The European civilization used and still uses two eras, B.C. ("Before Christ") and A.D. ("Anno Domini", Latin for "in the year of the Lord"). This system, which leads to a positive (forward) count for the A.D. era and a backward (negative) count for the B.C. era, has been widely accepted today. Some argue that in today's "global village" the terms B.C. and A.D. are offensive to civilizations not based on Christianity and propose the terms B.C.E. ("before the common era") and C.E. ("common era") instead. This does of course not eliminate the original problem that the European civilization imposes its own calendar on the world as the "common" era.
These lectures were developed by a member of the European civilization. While I have made every effort to give an unbiased view of the history of science and civilization, it would be futile to hide my cultural origins. Rather than imposing the Christian era as the "common era" on all civilizations I prefer to continue with the European tradition and use B.C. and A.D. to reflect my European upbringing. Whether my lectures will offend other civilizations will not be decided by the use of B.C. and A.D. but by their content.