All of the material so far in the Book of Mormon has come from the small plates of Nephi. Although the account in First Nephi evidently begins around 600 B.C., it is quite clear from 2 Nephi 5:28-33 that the small plates were not prepared until thirty years after Lehi left Jerusalem, or approximately 570 B.C. Thus, when Nephi started to write on the small plates in 570 B.C., he evidently started listing events in his earlier life, dating from the time his father left Jerusalem. This might be somewhat analogous to your starting this year a life history but writing first in your history some of the events of your childhood.