Alma declares that he does not know how soon the Messiah will come, even though the scriptures had predicted a rather specific number of years, and the countdown will prove that prophecy to have been accurate. As I read this, Alma was sharing our own difficulty with understanding when the scriptures are literal and when they are figurative. Alma surely knew of Lehi’s six-hundred-year prophecy but had not been able to discern whether that sum was specific or generic. Even had he known the year, however, he would not have known the day. Alma had to wait with faith.