According to Daniel Peterson, chapters 1-6 of Helaman cover fully thirty years and offer us a fairly good glimpse into how Mormon the editor worked. This section of the Book of Mormon is largely an example of what is often termed "annalistic" writing. "Annals," a type of historical record-keeping with many Near Eastern parallels, feature a narrative of events listed year by year. This is precisely what we find in these chapters. With only a single exception, everyone of the thirty years extending from the fortieth year of the judges up to the sixty-ninth (from 52 to 23 B.C.) is explicitly mentioned in these chapters.
As Mormon read through the chronicles that came to him, he knew he could include only a small portion in his abridgment. Therefore, under the inspiration of the Lord, he commented on the material he selected. However, he was too honest, too conscientious and thorough, to simply leave whole periods out of the record. For example, when he came to his sources for the years 43-50 B.C., he found a host of events and details that he could not put into his book. But he drew a lesson from them and in this case, the moral he distilled from his source chronicles is the evil of pride. [Daniel C. Peterson, "Their Own Worst Enemies," in Studies in Scripture: Book of Mormon, Part 2, pp. 92-93] [See the commentary on 4 Nephi 1:6]
Note* The fifty and fifth year is missing in the annalistic writings (see Helaman 4:1-4). Is that missing year significant? [Alan C. Miner, Personal Notes]