Here again we get a brief log of the years from the ninety-sixth year of the reign of the judge to the one hundredth year. They apparently passed uneventfully as far as useful recordable events were concerned, as before when the people “waxed strong in iniquity.” So, one-hundred years had passed since Mosiah established the system of judges. In addition, the record specifies that there had been nine years since the sign of the Savior’s birth, and 609 years since Lehi left Jerusalem.
This is a prelude to the noting of the change in calculating time. “The Nephites began to reckon their time from this period when the sign was given, or from the coming of Christ” (v. 8). Thus, from now on, the years given in the record were calculated from the birth of Christ.
Book of Mormon Central, “How Does the Book of Mormon Help Date the First Christmas? (3 Nephi 1:13),” KnoWhy 255 (December 21, 2016).