= Answer = AD stands for the Latin Anno Domini, meaning "Years" (Anno) of our "Lord" Domini and refers to the birth of Jesus which was about 2009 years ago. BC is before Christ. = Answer = Our Calender is a calender based on the sun and before 1582 has 365 1/4 days in a year. In 1582 Pope Gregory decreed a new calender of leap years but excluded the leap years on the century ie 1900 but put it back every 4 centuries ie 2000. It is a continuation of the Egyptian calender which dates back to about 327BC.