Since you're not going to read the info from the link I gave you, I'll post it here.
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Anno Domini, commonly abbreviated "A.D.", refers to the conventional numbering of years in the Gregorian calendar. It uses an epoch based on the traditionally reckoned year of the birth of Jesus Christ.
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Yet not even the Catholic Church claims that Pope Gregory's calendar starts from the birth of Jesus. Most historians agree that Jesus of Nazareth was born year 4 or 5 before the start of the year chosen as the start of the Gregorian calendar.
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So it looks like Christ was born 4 or 5 years before the commonly accepted start of A.D. That still doesn't explain if B.C ends when A.D. begins, but the gap wouldn't be 30 years.