Undoubtedly, 305 is a real date marking a specific event: the death of Amos2, son of Amos1 (A.D. 297).
Amos1 had died in 194 (v. 21). Thus, Amos2’s minimum lifespan was 106 years. To complicate this chronology, the record goes to his brother Ammaron—presumably his much younger brother. However, if that were true, then Ammaron would have been a minimum of 106, making Amos2 at least one year older.
There is no current explanation for this chronology that is fully persuasive. George Reynolds rather weakly suggested that this lengthy lifespan was a benefit of living the gospel:
There is one thing very noteworthy with regard to the descendants of Alma at this period, it is their longevity. Amos and his two sons (Amos and Ammaron) kept the records for the space of two hundred and ten years. This is a testimony to all believers in the Book of Mormon, to the highly beneficial results arising to the body as well as to the soul of every one who gives undeviating, continued obedience to the laws of God.
No people since the deluge, of whom we have any record, lived nearer to the Lord than did the Nephites of this generation; no people have had the average of their earthly life so marvelously prolonged.
Archibald F. Bennett agreed: “The above-named Nephi died A.D. 110, having kept the record seventy-six years. His son, Amos, kept it in his stead, for the long period of eighty-four years. It is evident that through righteousness the lives of these men were greatly prolonged.”
This explanation falters before the fact that scores of other righteous men did not live nearly so long; even the blessed disciples died at age seventy-two, which must have been a respectably old age (3 Ne. 28:3). It seems more likely to me that Mormon, in rapidly condensing three centuries, simply missed a generation. Adding in one generation would provide a much more reasonable lifetime. Although Mormon records no other instances of a grandson named for a grandfather, the only place with “room” for an extra generation is between the two Amoses, but this proposal has its own difficulties equal to the unusually long lives of Amos2 and Ammaron.
The question of whether the Lord would allow this kind of error in scripture is easily dealt with. All existing scriptures bear evidence of human error of one kind or another. Jesus’s “correcting” the Nephite records concerning Samuel’s prophecies provides scriptural evidence of such mistakes (3 Ne. 23:8–13). But would Mormon plausibly have made this error? I think so. He made what I argue is an interpretive mistake in the story of Ammon guarding the king’s flocks. (See commentary accompanying Alma 17:26ff.) Furthermore, he has shaped his history to present the Gadianton robbers as a continuing presence when it seems to have been historically implausible. (See Helaman, Part 1: Context, Chapter 3, “The Gadianton Robbers in Mormon’s Theological History: Their Structural Role and Plausible Identification.”) I have also argued that Mormon has written 4 Nephi to meet theological, not historical, goals. I therefore conclude that Mormon not only easily could have omitted a generation in his quick summary of the record’s transmission but did so. This scenario, though speculative, provides a more plausible explanation than positing preternaturally long lifespans for two record-keepers.
Chronology: In our calendar, Amos2 died in A.D. 298.