Ancient authors were often expected to provide a period or date for their writing. Moroni gave a date in Moroni 8:6, “Behold, four hundred years have passed away since the coming of our Lord and Savior.” There were twenty-one more years before he finally buried the plates, and it had been fifteen years since the final battle before Moroni was able to put this first conclusion on the record.
Four hundred is twenty squared, that is twenty times twenty. The Mesoamerican calendar system was based on the number twenty. One cycle of twenty years was called a tun and so it had been a tun of tuns. Moroni probably saw this as significant; otherwise, why would he have made particular mention of it? A good recordkeeper demarcates when and where the record was produced.
Mark Alan Wright, “Nephite Daykeepers: Ritual Specialists in Mesoamerica and the Book of Mormon,” in Ancient Temple Worship: Proceedings of the Expound Symposium, 14 May 2011, ed. Matthew B. Brown, Jeffrey M. Bradshaw, Stephen D. Ricks, and John S. Thompson (Salt Lake City and Orem, UT: Eborn Books and Interpreter Foundation, 2014), 252–253.
Evidence Central, “Book of Mormon Evidence: Calendrical Pattern,” September 19, 2020, online at evidencecentral.org.