Chronology: Omni’s first date is 276 years after the departure from Jerusalem. This would be 318 B.C. according to the lunar calendar.
Redaction: Omni clearly made entries at least twice, and perhaps three times. The first entry (vv. 1–2) may have concluded with the 276 year date or, perhaps, the beginning of the next 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.
It seems likely, however, that he wrote only twice, once 276 years after the departure from Jerusalem and again six years later just before he gives the plates to his son. Thus, he apparently had the plates for forty-five years before he made his first entry. This would not be unusual, given the performance of his ancestors from Enos on, and especially since Omni obviously did not consider himself an expert on spiritual matters.
To speculate, he may have decided to make his first entry after being seriously wounded (since he declares himself a warrior), but he would have been older than forty-five by perhaps ten years—surely an advanced age for a man to still be engaged in warfare. When he makes his final entry six years later, he is presumably near death and considers that he has fulfilled his charge to “keep the plates”—not necessarily to record, but to preserve.
History: Using the 586 B.C. departure date, Omni writes between 318–312 B.C. In highland Guatemala, this period corresponds to the Middle Preclassic (or “Miraflores”) period when religious architecture flourished in Kaminaljuyú. This architecture is Mesoamerican, however, not Israelite. Instead of menorahs and horned altars, archaeologists find world trees and effigy altars. However, the process of iconic assimilation would have occurred much earlier in Nephite history, I suspect within a decade after arrival.
How then should we see the people of Nephi in this context? The Nephites are now acculturated and have adopted the artistic traditions of the host culture’s architecture and pottery. With the posited increase of trade and the resulting influence of external ideas, Jacob’s warning speech makes sense, given the social and archaeological context in highland Guatemala for this time.
Accommodation to the physical culture need not preclude the continuation of Israelite religion. Modern Latter-day Saints adorn their homes with Christmas trees that originated with pagan rites and have no trouble recontextualizing them as an acceptable part of Mormonism. Indeed, given the absence of the typical Christian crucifix, it might be difficult to identify LDS homes in an archaeological record from those of neighbors of other religions. Photographs of temples and the presence of scriptures and Church magazines would probably not survive long enough for archaeologists of the future to identify a Mormon sub-strand in the larger culture.