The calculation involves the time around A.D., which is different from simply calculating the time after B.C. or A.D., that is, one year must be subtracted from the calculated total time.

This is a point that should be pointed out when using advertising calendars.

For example, if we calculate the number of years between 84 1 and 1949 BC, the correct calculation is 841+1949-1= 2789. This algorithm can be summarized as a simple formula "Add before and after, and subtract one".

The reason why we want to reduce one year here is that the year of A.D. has not been designated as one year, so we can't calculate the time around A.D. according to the mathematical concept of positive and negative numbers.

Extended data

The year when Jesus was born is the beginning of a year. In julian calendar and Gregorian calendars, the date after the birth of Jesus is called the principal year, Anodomini (AD) (Latin). Before the birth of Jesus, it was called "BC".

However, in order to dilute its religious color and avoid causing antipathy among non-Christians, modern scholars mostly change its name to Common era (abbreviated as C.E.) before Common Era (abbreviated as B.C.E).

References:

Baidu encyclopedia advertising chronology