Time before the NT was consider the BC period, or Before Christ, the years of B.C. go up in number as one goes back in time of the birth of Jesus. So if something occurred 400 years before the birth of Christ it would be considered 400 B.C.
At the birth of Jesus, the year was considered 0 and the years thereafter began increasing in number and are AD which means in the year of the LORD.
A year is still a year. So being 2015 A.D., it was 2015 years since the time of Jesus was born.