Why do historians use terms such as A. D., B. C., decade, and century

i see, would you mind putting it as one?

ewr rr r r r r r r r rrr

most people such as me i dont have one but if   i did i would mostly likely quit my job if the economy was down cylcling that means less money if you go to a bigger job that is prospering you can earn more wage and still be able to feed yourself or why you work all people want more money then they have guarteed most people like me would switch jobs for a better one with higher wage