It’s complicated actually but this is something you might consider?
https://www.christianity.com/wiki/jesus-christ/when-was-jesus-born.html
What Year Was Jesus Born?
Confusion in Historical Calendars
Several factors are involved in establishing the year of Christ’s birth. There were two dating systems at the time of Julius Caesar in AD46. One was his new Julian calendar and the other calendar began with the year Rome was established: 753BC. To complicate matters, Julius Caesar also determined that a year would be calculated beginning with “the accession of absolute power by the then emperor,” according to BibleStudyTools.com.
About 500 years later, “a mathematically-minded monk…Dionysius Exiguus invented the concept of AD,” BibleStudyTools.com stated. His purpose was to establish when Easter should be celebrated by working backwards using a complicated system of his own. He decided that Christ was born in AD1, and his system was adopted in Europe around 200 years later.