As the final battle nears, Mormon “began to be old”—a classic Book of Mormon statement that his life is nearing its end. (See commentary accompanying Jacob 1:9.) He makes preparations for his demise. An essential preparation is transferring his book to Moroni. But since Mormon writes again on these very plates, the exchange obviously did not take place at this precise pre-battle moment.
Mormon’s statement that he “made this record out of the plates of Nephi” may sound as if he wrote what we know as the Book of Mormon during the four years of preparation. The evidence suggests that he was at work on it earlier than the beginning of these four years. Mormon explicitly tells us that he was writing on the abridgment five years earlier, in the 380th year. (See commentary accompanying Mormon 5:8–9.)
We can reconstruct a basic history of Mormon’s work with the Nephite records. In the 345th year, Mormon removed the plates of Nephi from the hill Shim (Morm. 2:16–17). Mormon doesn’t tell us the extent of these plates he removed at this time, but it is improbable that they were the full set of the “plates of Nephi.” He explicitly tells us that these were the very plates on which other record-keepers had written (see Morm. 2:18). At this time, he tells us that “upon [these] plates of Nephi I did make a full account of all the wickedness and abominations.”
We next see the full collection of plates in the 367th year when Mormon retrieves the plates from the hill Shim because the land is being overrun by the Lamanite/Gadiantons, and he wants the full set of plates to be safe (Morm. 4:23).
The plausible scenario, then, has Mormon writing as a chronicler or regular record-keeper for at least twenty-two years on the large plates of Nephi. He could not compose his own book without access to the records from Shim—and he apparently did not acquire them until the 367th year.
For the next thirteen years, he read and digested them, possibly taking notes and preparing an outline or rough draft. He then began writing the work we have as our Book of Mormon at least between 380 and early 385 but wrote at least our Chapters 6–8 after the final battle.