According to Warren and Palmer, the reference to "nine moons" (Omni 1:21) is in harmony with our knowledge concerning the ancient calendar systems in Mesoamerica. Lowe et.al. (1982) make clear that in each of the sixteen Maya languages in the lists of Stoll (1958:77-78):
. . . the word for moon is also the word for month; the same is true of the Popoluca, Zoque and Mixe lists in Bom and La Farge (1926-1927, 2:461). Such consistency in linguistic parallelism over so wide an area (when included with Central Mexico) not only confirms that "moon" and "month" are indistinguishable but establishes the additional fact that this indistinguishability is very ancient.
[Bruce W. Warren and David A. Palmer, The Jaredite Saga, unpublished]
“He Dwelt with Them for the Space of Nine Moons”
In Omni 1:21 we find the term "moon" used to denote time:
And it came to pass in the days of Mosiah, there was a large stone brought unto him with engravings on it . . . and they gave an account of one Coriantumr . . . and Coriantumr was discovered by the people of Zarahemla; and he dwelt with them for the space of nine moons. (Omni 1:20-21)
Because this is the only time the term "moons" appears in the Book of Mormon as a unit of time measurement, it is hard to tell whether it was part of the Mulekite calendar (used only by the people of Zarahemla) or whether the Nephites also used a "lunar" calendar.
However, according to Randall Spackman, the fact that Amaleki used this term "moons" around 200 B.C., which was about four centuries after Lehi left Jerusalem, is evidence that both the Nephites and the people of Zarahemla shared a basic understanding of lunar observation, dividing time into moons, and adding moons to mark off longer periods of time.
Mormon, the Nephite abridger of the Book of Mormon record, used the words "month" and "months" when he wrote the books of Alma and 3 Nephi after about A.D. 350. Although neither author (Amaleki or Mormon) defined the calendrical terms "moons" and "months," the appearance of the two English cognate terms in the book of Mormon translation implies that the underlying Nephite words had different meanings, however slight.
Although the terms "moons" and "months" might be referring to essentially similar periods, we might some subtle reasons for using two different terms. [Randall P. Spackman, "The Jewish/Nephite Lunar Calendar," in Journal of Book of Mormon Studies, Vol. 7/1 , November 1, 1998, p. 55]
Note* While Amaleki does not state specifically whether the Nephites used a lunar calendar, it is quite probable that the people of Zarahemla who reported the find of Coriantumr at least reported something related to "lunar" time in order for Amaleki to record the term "moons."
It is also an oddity that the terms "month" and "months" only occur in the books of Alma and 3 Nephi (Alma = 10; 3 Nephi = 4). It is also very significant that in at least one instance (3 Nephi 8:5) mention of the term "month" is not only made with the term "years," but the exact years after the sign of Christ's birth in which the great destruction happened. In this instance, the years and months cannot be accounted for by a strictly lunar calendar (A twelve-moon calendar averages only 354.367 days per year, eleven days fewer than a solar calendar year, which averages 365.2422. By the time Christ reached the passover beginning his 34th year, having completed 33 years, he would be 33 X 11 days, or 363 days, or almost 1 solar year behind. Mormon distinctly mentions that after the sign of Christ was given, "the Nephites began to reckon their time from this period when the sign was given, or from the coming of Christ" (3 Nephi 2:8), however nothing at all is said relative to the a different length for the months or years.