In his editing of this passage for the 1837 edition, Joseph Smith replaced two instances of the relative pronoun which with when; in standard English we expect when as the relative pronoun when the referent involves time. As explained under 1 Nephi 19:2, the original text of the Book of Mormon sometimes used which (possibly as a Hebraism) in place of the expected when. Here in 3 Nephi 2:7–8, the critical text will restore the two original which ’s that refer to time.
Summary: Restore in 3 Nephi 2:7–8 the two original uses of which as the relative pronoun; although standard English prefers when in these two cases, the original text of the Book of Mormon allows for either when or which.