As explained in some detail under Mosiah 8:17, the text distinguishes between past and passed. When referring to something as being over or finished, the correct form is the adjective past; thus here in Mormon 2:15 the word should be past: “the day of grace was past with them”. Similar instances of past in the text support this spelling:
As shown under Mosiah 8:17, the scribes frequently spelled the adjective past as passed. Here in Mormon 2:15, scribe 2 of 𝓟 wrote the word as passed, but the 1830 edition has the correct past. Yet the 1849 LDS edition reintroduced the spelling passed in this passage, with the result that all subsequent LDS editions have retained passed rather than the correct past. This 1849 change was not implemented for other instances of past in the text. The change seems to have been the result of a momentary decision, either by the 1849 typesetter or by Orson Pratt (the editor for that edition). Paul Thomas was the first one who brought to my attention this spelling difficulty in Mormon 2:15.
Don Brugger points out (personal communication) that these four examples of past could be considered instances of passed if we interpret the be verb as the archaic perfect auxiliary. As explained under 2 Nephi 22:2 and Helaman 13:36, verbs of motion and change in earlier English took the be verb rather than have as the perfect auxiliary (thus the archaic “he is risen” rather than the modern “he has risen”). By means of this interpretation, one could argue that all four of these Book of Mormon instances of “to be past” could be read as “to be passed”:
Of course, historically the original source for “to be past” is the archaic “to be passed”, where past is a spelling variant for passed. Yet even the King James Bible prefers past when the meaning is ‘over’, as in the following examples:
Even so, the corresponding synoptic passage in Mark for the Matthew example reads as passed:
But the original Greek for this Mark example is adjectival and could be literally translated as ‘and now the hour [is] much’. (Interestingly, the Greek equivalent for “is past” in the Matthew passage is verbal, not adjectival.) In accord with the normal King James usage, the critical text will use past for all four of the Book of Mormon instances where the meaning appears to be adjectival rather than verbal.
Summary: Restore in Mormon 2:15 the 1830 spelling past since the meaning here is ‘over’.