Question

why instead of BCor AD we add circa with some dates

Answer 1

stands for anno domini, Latin for “in the year of the lord,” and refers specifically to the birth of Jesus Christ. ... In recent years, an alternative form of B.C./A.D. has gained traction. Many publications use "C.E.," or "common era," and "B.C.E.," or "before common era."May 9, 2014