Historical: Omni's first date is 276 years after the departure from Jerusalem. This would be 310 BC.
Textual: Omni clearly writes at different times. From the breaks in the text, we may suggest that he wrote at least twice, and perhaps three different times. The first entry is the easiest, and comprises at least our current 1 and 2. This first entry may have concluded with the dating given at the beginning of verse three, or that dating might indicate a separate entry. The last entry is clearly the one beginning with the final date, as there is no reason in the text for the two dates to have been given at the same writing.
With the little information we receive from Omni, it is very likely that there were only two occasions when he wrote, the first being 276 years after the departure from Jerusalem, and the second 282 years after, at which time he apparently sees the end of his life coming, and gives over the plates to his son.
The last date given in Jarom is 238 years after the departure from Jerusalem, at which point the transfer of the plates occurs. We may suppose, therefore, that Omni waits 45 years before making his first entry onto the plates. This would not be unusual given the process we have seen from Enos on, and especially when combined with Omni's admission that he has not kept the statutes as he ought. Since these plates were to record the spiritual history of the Nephites, Omni clearly did not see himself in a position to expand on spiritual matters.
We haven't sufficient information to even speculate what occasion transpired to urge Omni to his first recording after 45 years. One might suspect that with the declaration of his experience in combat that perhaps he was injured at thought it wise to make a record just in case - however he would have been older than 45, which would make it rather unusual for a war wound. For whatever reason, Omni does write 45 years after receiving the plates, and does not make a final entry at that time. He closes his entry 6 years later. At that point we may safely presume that he viewed his mortality as imminent. In any case, Omni considers that he has fulfilled his charge to "keep the plates." For Omni, the charge is not so much to write, but to preserve, which thing he did.
History: Using the 586 BC date for the departure from Jerusalem, we have Omni writing in 310 BC to 304 BC. In highland Guatemala at this time we are in the Middle Preclassic time period. In highland Guatemala, this is known as the Miraflores phase. It is during this period that we find an increase in the religious architecture in Kaminaljuyu.
What must be understood from the archaeology, however, is that the evidence of religion is not evidence of Israelite religion. The religious architecture and artifacts reflect the known religious trends in Mesoamerica. The imagery is Mesoamerican, not Israelite.
How then should we see the people of Nephi in this context? I suggest that the underlying currents in the Book of Mormon are completely consistent with this picture. The Nephites are being acculturated to their surroundings, and are picking up the artistic trappings that go with the area's conceptions of what architecture and pottery should be like. With the posited increase of trade connections with other people, and thereby the influx of external ideas, Jacob's speech makes a lot of sense in the context of the physical catalog of the peoples of highland Guatemala at this time.
Of course this requires that we understand that the accommodation to the physical culture need not preclude the continuation of the revealed religion among them. Modern LDS use Christmas trees that trace their origins to pagan rites, and we are quite able to recontextualize them into an acceptable part of our religion. Indeed, in the absence of the typical Christian crucifix, LDS homes might be hard pressed to find physical imperishable artifacts that clearly delineate us as LDS as opposed to the religion (or non-religion) of any of our neighbors.