In Doctrine and Covenants 95:7 we read:
And for this cause I gave unto you a commandment that you should call your solemn assembly, that your fastings and your mourning might come up into the ears of the Lord of Sabaoth, which is by interpretation, the creator of the first day, the beginning and the end. (emphasis added)
Matthew L. Bowen notes how this is a direct hit for Joseph Smith:
When considered in its entirety, this revealed gloss is right on target. The creation/begetting of the heavenly hosts was associated with “the first day” or “Day One” in ancient Israelite thought. They are described as “finished” or fully prepared by the end of the six creative periods (“days” in Genesis 2:1). Additionally, “Lord of Sabaoth” or Yhwh ṣĕbāʾôt is to be understood in connection with the similarly constructed name-title Yhwh ʾĕlōhîm (“He creates gods,” “he causes gods to be,” or “he brings to pass gods”). The meristic appositive title “the beginning and the end” implies that Yhwh is not only the “author”/“creator” of Israel and its salvation but the “finisher” thereof. Far from evidence of Joseph Smith’s lack of knowledge of Hebrew, the interpretive gloss in D&C 95:7 constitutes evidence of Joseph’s ability to obtain correct translations and interpretations through revelation.– Matthew L. Bowen – “Creator of the First Day”: The Glossing of Lord of Sabaoth in D&C 95:7
Where would Joseph Smith have learned this meaning?