John W. Welch, Donald W. Parry, and Stephen D. Ricks have noted how the Book of Mormon uses the phrase “this day” 18 times, most of which are in a formal setting where people are gathered together.
King Benjamin uses the phrase 5 times in his address, such as in Mosiah 2:15
Yet, my brethren, I have not done these things that I might boast, neither do I tell these things that thereby I might accuse you; but I tell you these things that ye may know that I can answer a clear conscience before God this day.
This usage of the phrase is consistent with its use in the Bible, especially in its connection to the temple or making covenants.
How would Joseph Smith have noticed this in the Bible if most readers don’t notice it after a lifetime of study?