Dr. Mark Wright noticed that the Nephites did not immediately begin to mark time from the sign of the Messiah’s birth, but rather back-dated that new beginning of their calendar only after 100 years had passed in the reign of the judges. He suggested that the Nephites paid attention to sets of years, and preferred to deal with time in more symbolic units. Therefore, even though there was a great rebirth of belief at the time of the sign, the change of the calendar to mark that time came later, only after one hundred years of the reign of the judges had passed.
Modern readers understand the change of a calendar to mark Christ’s birth, as we follow a calendar that uses that event as a beginning point. In the Book of Mormon, it was the third time that there was a change in the beginning point of the calendar. The first was when Nephite time was counted from when Lehi left Jerusalem, which was a change from the calendar used in Jerusalem at that time. The second was with the change from monarchy to the reign of the judges.