The term "earth" as is used in Helaman 14:27 referred only to the land in which the Book of Mormon peoples lived. It wasn't a reference to the "world". I think this is even more proof that the Book of Mormon is true and that Joseph Smith didn't write it himself. He knew the bible very well and would have immediately recognized the inconsistency if he were writing it himself. Because he was translating it from ancient authors, he didn't give it a second thought.

I found this article, and thought it was interesting. 
 
"The "mists of darkness" apparently refer to volcanic ash that was present at this time in Central America (Book of Mormon territority) but not in Jerusalem. Many of the literally earth-shaking events described in the Book of Mormon at the time of Christ's death are consistent with what is now known about volcanic activity - quaking ground, cities being buried and burned, changes in water level or tidal waves associated with seismic activity (some cities were sunk), and intense lightning and storms. Most particularly, volcanic activity can result in thick volcanic ash that meets the descriptions of the "mists of darkness" in the Book of Mormon (3 Nephi 8:19-23)): the ash can be felt, it can overpower people (3 Nephi 10:13), it can make it difficult to light fires (especially when the ash is moist or is associated with rain), it can block out sunlight completely, and it can persist for days (three days, for example). Volcanic activity is strongly implied in the text and is accurately described (and Joseph Smith could not have known such things himself). The only question, then, is whether there is any evidence for volcanic activity in the New World at the time of Christ's death. The amazing answer is yes: there was significant volcanic activity in the New World near the time of Christ's death - and the location is Central America, the limited area that serious LDS scholars have concluded must be the region described in the Book of Mormon." 
 
Here is the link to where I found it. 
 
http://www.lightplanet.com/mormons/response/qa/bom_darkness.htm

The hours referred to in Matthew are the three hours that Jesus was upon the cross. At this time there were terrible tempests, earthquakes, fires and other natural disasters upon the face of the Americas. The three days, in contrast, are referring to the time that Jesus was in the tomb. 
 
Luke 24:7 
"...The Son of man must be delivered into the hands of sinful men, and be crucified, and the third day rise again."