Chronology. The fifteenth year of the reign of the judges would be approximately 79 BC in the correlation used in this commentary.
We have three recorded dates for two military actions. The first date is from Alma 16:9 that tells us that the destruction of Ammonihah takes place in the eleventh year of the reign of the judges. We then discover that there are three years of peace, and that the Lamanites return in the fourteenth year (Alma 16:12). The battle of the fourteenth year of the reign of the judges would be the same as the one we have just now seen, yet we have here the ending of the fifteenth year of the reign of the judges. This might appear to indicate that the hostilities covered an entire year.
This is unlikely to be the case. Conflicts would have been very difficult to sustain over that length of time for logistical reasons alone. The attacking army would require food, and a year-long battle would give neither attacker nor defender the opportunity to plant and harvest. What we must therefore understand is the “missing year” between the attack that comes in the fourteenth year and this statement that the fifteenth year has ended.
We can conclude that the battle has terminated prior to the end of the fifteenth year because Mormon includes the indications of mourning prior to closing the year record. This lets us know that we do not need to see the battle as extending to the end of the fifteenth year. What we do not know is how close. This may be an irrelevant question, however, given the nature of the record that Mormon appears to be using.
Mormon’s source appears to be broken into year records. At the end of the events of the year, the year record closes. Within that year, we do not always get an accurate picture of the timing of events, perhaps because it was uninteresting to Mormon in his distant timeframe and religious purpose in creating the record. What we have here is Mormon simply closing the year as his source does. While we might appear to have a missing year, it is most likely that this is simply an artifact of the way Mormon uses his source material.