A.D. Chronology means that the year when Jesus was born is the first year of A.D., and the year before it is one year BC. Therefore, when calculating how many years there are before and after A.D., we can't subtract two numbers on the number axis. There is no coordinate "zero" on the "number axis" in A.D., so there must be a number to fill this position when calculating. Then, this problem is expressed by a mathematical formula: 6-(-78)- 1 = 83 (assuming BC is negative and AD is positive), and the negative sign of 1 is "complement". If a person was born in the first year of BC, he would be only one year old by the first year of AD. If all the people were born and died in BC or after AD, there would be no such problem. Only those who were born in BC and died after AD have this problem.