Section 110 was not published in the D&C until 1876, although it had appeared in print in 1852. There is some doubt that all of this revelation came from Joseph Smith. Specifically notice what D&C 110 says and then notice what Joseph has to say about Elijah after 1836.
D&C 110: 13-16
13 After this vision had closed, another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—
16 Therefore, the keys of this dispensation are committed into your hands; and by this ye may know that the great and dreadful day of the Lord is near, even at the doors.
“The vision was recorded, but as of yet, there is no evidence that the vision was publicly taught by Joseph Smith nor by Oliver Cowdery.” (BYU Master’s Thesis by Trever R. Anderson)
Statements from Joseph Smith after 1836 indicate that Elijah had not come yet and restored keys. However, we might be misunderstanding Joseph’s words. Is it possible that Joseph is “speaking in the future tense to explain the scriptures to his audience? Meaning he placed himself linguistically in the time of the ancient prophet Malachi who was prophesying the return of Elijah in the future, to explain what had already occurred. Or he was simply quoting the scripture written thousands of years ago with future tense language.” (suggested by Michael)
October 5 1840
Elijah was the last Prophet that held the keys of this Priesthood, and who will, before the last dispensation, restore the Authority and delive[r] the keys of this Priesthood, in order that all the ordinances may be attended to in righteousness. It is true that the Savior had authority and power to bestow this blessing; but the sons of Levi were too prejudi[ced]. And I will send Elijah the Prophet before the great and terrible day of the Lord, etc., etc. Why send Elijah Because he holds the Keys of Authority to administer in all the ordinances of the Priesthood; and without the authority is given, the ordinances could not be administered in righteousness. (Words of Joseph Smith, p. 43)
August 1843
The world is reserved unto burning in the last days. He shall send Elijah the prophet, and he shall reveal the covenants of the fathers in relation to the children, and the covenants of the children in relation to the fathers. (TPJS p. 321)
How shall God come to the rescue of this generation? He will send Elijah the prophet. The law revealed to Moses in Horeb never was revealed to the children of Israel as a nation. Elijah shall reveal the covenants to seal the hearts of the fathers to the children, and the children to the fathers. (TPJS p. 323)
October 3, 1841
The dispensation of the fulness of times will bring to light the things that have been revealed in all former dispensations, also other things that have not been before revealed. He shall send Elijah the prophet &c., and restore all things in Christ. (Words of Joseph Smith, p. 79)
January 1844
The Bible says, I will send you Elijah before the great and dreadful day of the Lord Come that he shall turn the hearts of the fathers to the children & the hearts of the Children to their fathers lest I Come & smite the whole earth with a Curse,‘ Now the word turn here should be translated (bind or seal) But what is the object of this important mission or how is it to be fulfilled, The keys are to be delivered the spirit of Elijah is to Come, (Words of Joseph Smith, p. 318)
December 1898 edition of the Improvement Era. An article about Oliver Cowdery by Elder Samuel W. Richards, a close friend of Oliver.
“On April 3rd, 1836, he was favored, with the Prophet Joseph, to witness the marvelous manifestations which occurred in the Kirtland Temple, when they saw the Lord standing upon the breastwork of the pulpit, and received from Him the declaration that their sins were forgiven them, and that they were clean in His sight. Immediately after this, also appeared in succession Moses, Elias, and Elijah, each delivering up the keys and powers of their several missions and dispensations to Joseph and Oliver and while standing in their presence declared the time had come for the turning of the hearts of the fathers to the children and the children to the fathers, lest the whole earth be smitten with a curse; and the keys of this dispensation were committed to them by the several administrators who had held them in former dispensations.”
Warren Cowdery (Oliver’s brother) wrote the account that we have in D&C 110. He left the church in 1838 and continued to live in Kirtland where he died in 1851. He would have written the account prior to 1838 when he left the church. Warren was Joseph Smith’s scribe in 1836 when he would have received a dictation of the account and transcribed it in Joseph’s 1836 journal.
In 1839, Joseph began the Manuscript History of the Church. When Joseph died in 1844 the history was complete up to August 5th, 1838. So, the entry for April 3, 1836 would have been done under Joseph’s direction.
“The vail was taken from our minds, and the eyes of our understanding were opened; we saw the Lord standing upon the breast work of the pulpit. before us, and under his feet was a paved work of pure gold, in color like amber. His eyes were as a flame of fire; the hair of his head was <white> like the pure snow his countenance shone above the brightness of the sun, and his voice was as the sound of the rushing of great waters, even the voice of Jehovah, Saying. I am the first and the last. I am he who liveth, I am he who was slain, I am your advocate with the Father. Behold your sins are forgiven you. You are clean before me, therefore lift up your heads and rejoice, let the hearts of your brethren rejoice,— and let the hearts of all my people rejoice. who have with their might, built this house to my name. For behold I have accepted this house, and my name shall be here; and I will manifest myself to my people, in mercy, in this House, yea I will appear unto my servants, and speak unto them with mine own voice, if my people will keep my commandments, and do not pollute this holy house, yea the [p. 727]
ye[a] the heart of thousands
and
tens of thousands shall greatly <April 3.> rejoice, in
consequence of the blessings which shall be poured out, and the
endowment with
which my servants have been endowed in this house. And the fame of this
house
shall spread to foreign lands, and this is the beginning of the
blessing, which
shall be poured out upon the heads of my people even so, Amen. After
this
vision closed the heavens were again opened unto us, and Moses appeared
before
us, and committed unto us the keys of the [HC 2:435] gathering of Israel
from
the four parts of the earth, and the leading of the ten tribes from the
land of
the north. After this Elias appeared and committed the dispensation of
the
gospel of Abraham, Saying that in us and our
seed all
generations after us should be blessed. After this vision had closed,
another
great and glorious vision burst upon us, for Elijah, the Prophet, who
was taken
to heaven without tasting death, stood before us and said behold the
time has
fully come, which was spoken of by the mouth of Malachi, testifying that
he
should be sent, before the great and dreadful day of the Lord should
come, to
turn the hearts of the fathers to the children, and the children to the
fathers
lest the whole earth be smitten with a curse. Therefore the keys of this
dispensation are committed into your hands and by this you may know,
that the
great and the dreadful day of the Lord is near, even at the doors.”
(History, 1838–1856, volume B-1 [1 September 1834–2 November 1838], p.
727, The
Joseph Smith Papers, accessed April 15, 2024, https://www.josephsmithpapers.org/paper-summary/history-1838-1856-volume-b-1-1-september-1834-2-november-1838/181)
A useful video on Elijah’s visit to the Kirtland Temple.