Question: How many years passed before the Nephites incorporated a retroactive dating system based on the appearance of the new star?
Answer: Nine years after the sign of Christ’s birth appeared, the Nephites began a retroactive dating system that stemmed from the sign of the star. There were three calendar shifts noted in the Book of Mormon.
The first calendar shift began when Lehi left Jerusalem in 600 B.C. and lasted until the end of the reign of monarchs in 92 BC (1 Nephi 10:4).
The second calendar shift began with the reign of judges in 92 BC and lasted until a retroactive dating system, based on the appearance of the new star, was employed in AD 9 (see Mosiah 29:44: Alma 1:1).
The third calendar shift dated from the birth of Christ (30 AD) until Moroni ended his writings in AD 421 (3 Nephi 2:5-7).
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 174.