"The Nephites used three systems of reckoning time:
(1) The first method was to determine the number of years since father Lehi left Jerusalem; this system was used from 600 B.C. to 92 B.C. (Mosiah 29:44-47; Alma 1:1.)
(2) The second method was to calculate the number of years from the beginning of the reign of the judges; this system was used for about 100 years, from 91 B.C. to A.D. 9. (3 Nephi 2:5-7.)
(3) The last method was to 'reckon their time from this period when the sign was given, or from the coming of Christ' (3 Nephi 2:7-8); this system was used for the remainder of their record." (Daniel Ludlow, A Companion to Your Study of the Book of Mormon, p. 254)