Dates in the Book of Mosiah

I recently described what we know of the chronology of the Book of Mosiah.  As I did so one of the perplexing things to me was where the current edition of the Book of Mormon came up with the dates for some of the events, in particular the departure of Zeniff from Zarahemla placed at about 200 BC and the first appearance of Abinadi in about 150 BC.  As far as I can tell, though those are plausible guesses, the text does not give us that indication.  In fact, the only years we have specifically given in the text around this time are these:

•  Amaron wrote in the small plates about 280 BC: “Behold, it came to pass that three hundred and twenty years had passed away” (Omni 1:5).
• King Benjamin gave his famous sermon and announced that his son Mosiah II would be king in 124 BC: “And Mosiah began to reign in his father’s stead. And he began to reign in the thirtieth year of his age, making in the whole, about four hundred and seventy-six years from the time that Lehi left Jerusalem” (Mosiah 6:4).   
• In 121 BC (“three years” after the start of his reign), Mosiah II sent a group of 16 “strong men” including Ammon to go and find their brethren who had left with Zeniff (Mosiah 7:1-2).

Thus it would appear that Limhi and his people made it back to Zarahemla around 121-120 BC.  But we don’t know when the first Mosiah traveled from the land of Nephi to the land of Zarahemla, when he died and King Benjamin started his reign, or at what point Zeniff and his group left to go up to the land of Nephi.  Here are a few other clues that we do have about this time: 

• Amaleki, the grandson of the brother of Amaron, “was born in the days of Mosiah” and he “lived to see his death” and the start of the reign of King Benjamin (Omni 1:23).  He was also alive when Zeniff’s group left from the land of Zarahemla.  That means that they could have left either during the time of Mosiah I or at the start of the reign of King Benjamin.  Amaleki actually mentioned the event in his record after he wrote about the start of King Benjamin’s reign, though he didn’t clarify the order of those two events. 
•        Zeniff recorded that he possessed the land of Nephi for at least 34 years: 12 years until their first battle with the Lamanites and then 22 years of peace after that until they fought again (Mosiah 9:11, 10:3). 
• The only indication about time that we have with King Noah is that Abinadi came, left for two years, and then came back (Mosiah 12:1).
• Alma (the Elder) died in about 91 BC at the age of 82 and would have been about 53 years old when his people made it back to Zarahemla from the land of Helam (per Mosiah 29:45-46).  But that doesn’t help us much because we don’t know where he was born or what his age was at the time he was a priest of Noah.    

From this, we know that the length of time from the departure of Zeniff to the return of the people of Limhi was at least 34 years plus the time King Noah reigned (at least two years) plus the time that King Limhi governed the people during their bondage.  We know that this period covered the first few years of the reign of Mosiah II, most or all of the reign of King Benjamin, and possibly some of the reign of Mosiah I depending on when they left.  The dates assumed in the text puts the length of this period at 80 years (with Zeniff leaving in 200 BC and Limhi returning in 120 BC), but that seems too long to me for a couple of reasons.  First, it would make the reign of King Benjamin almost that long—that’s possible but that would be surprising, especially since his son only reigned for 33 years.  The other clue we have I believe is the fact that three years into Mosiah’s reign, “he was desirous to know concerning the people who went up to dwell in the land of Lehi-Nephi, or in the city of Lehi-Nephi; for his people had heard nothing from them from the time they left the land of Zarahemla; therefore, they wearied him with their teasings” (Mosiah 7:1).  If it had really been 80 years since they left Zarahemla, that would be quite surprising that suddenly they became very interested in learning the fate of some group who very few if any had ever known.  It would make more sense to me if it had been on the other hand more like 40 or 50 years since the departure, with some of the older generation remembering personally those who left with Zeniff’s group.  That would also then fit better as the approximate length of King Benjamin’s reign.        
               Of course, the dates of this period are not that important and Mormon didn’t see the need to give us a lot of those details.  And I certainly may be missing some other clue in the text which validates the dates in the chapter headings as they exist.  What is important, and what Mormon wanted us to see through the story of these Nephites in the land of Lehi-Nephi, is that when we are righteous the Lord will bless and strengthen us.  And when we are wicked and choose to follow wicked rulers like King Noah, His judgments will ultimately come upon us.