In this post, Lord Wilmore demonstrates that angels are the source of the name of Christ. I just realized that angels also are the source of the timing for the coming of Christ. This is special, sacred knowledge not normally revealed, except in special circumstances by holy angels. Considering all cases of the timing of the coming of Christ in the Book of Mormon certainly leads to this conclusion. The first time this is brought up in the Book of Mormon is 1 Nephi 10:4
Yea, even years from the time that my father left Jerusalem, a would the Lord God raise up among the —even a , or, in other words, a Savior of the world.
There is no mention of any angel here. But in 1 Nephi 19:8, Nephi repeats this and reveals the source of information
And behold he , according to the words of the angel, in years from the time my father left Jerusalem.
2 Nephi 25:19 may be more ambiguous
For according to the words of the prophets, the cometh in years from the time that my father left Jerusalem; and according to the words of the prophets, and also the word of the of God, his shall be Jesus Christ, the .
Here we have the source of both the name of Christ and the timing of his coming. The timing is "according to the words of the prophets", but "the word of the angel" is added indicating where the prophets got their knowledge in the first place.
Even though the 600-year prophecy is repeated a few times, there is no count-down in the Book of Mormon. I am speculating here, but I believe that this was information only contained on the small plates and not generally revealed and distributed to the people. The time of Christ's coming is only mentioned in unclear terms after the Book of Omni when the small plates were full and only the large plates continued. Not until Samuel the Lamanite prophesied 5 years before his coming, was a specific time mentioned again.
King Benjamin brings it up without a specific time stamp but with the assurance that these are the words of an angel.
2 And the things which I shall tell you are made known unto me by an from God...
5 For behold, the time cometh, and is not far distant, that with power, the who , who was, and is from all to all eternity, shall come down from heaven among the children of men, and shall dwell in a of clay...
The topic is brought up again in Alma 13:24-25
24 For behold, are declaring it unto many at this time in our land; and this is for the purpose of preparing the hearts of the children of men to receive his word at the time of his coming in his glory.
25 And now we only wait to hear the joyful news declared unto us by the mouth of angels, of his coming; for the time cometh, we how soon. Would to God that it might be in my day; but let it be sooner or later, in it I will rejoice.
This one might seem strange. Alma says he does not know when Christ will come. Had he not read the small plates? A careful reading suggests that Alma was talking about Christ's coming to the Nephites. The 600 year prophecy was about Christ's coming to earth, i.e. his birth at the land of their fathers. In verse 24 above, we read about the people in "our land" receiving "his word at the time of his coming in his glory". This must be referring to his appearance to the Nephites. Alma probably knew from 2 Nephi 26:1 that it would happen sometime after his death and resurrection, but he didn't know when. Even though he probably knew about the 600 year prophecy, he was not giving away too much.
Another puzzling statement here is "would to God that it might be in my day". If Alma knew that Christ's coming to earth was yet 82 years away and Christ would appear to the Nephites after that, how could it possibly be in his day? The answer is simple: Alma knew that was not the case. "Would to God" in the Book of Mormon means "I know it is not the case but I wish it were". This expression is used twice elsewhere in the Book of Mormon (Jacob 1:8 and Alma 39:7) and we can see from both of those verses that this is what is meant.
Then, finally, we get to Samuel the Lamanite.
2 And behold, he said unto them: Behold, I give unto you a sign; for years more cometh, and behold, then cometh the Son of God to redeem all those who shall believe on his name...
9 And behold, thus hath the Lord commanded me, by his angel... (Helaman 14)
I might have missed something, but as far as I know, these are all the occasions where the Book of Mormon mentions the time of the coming of Christ. Every time, angels are the source of information. They are messengers bringing sacred knowledge in special circumstances. Before the coming of Christ, this sacred knowledge included his name and the time of his coming.
Matthew 24:36 is interesting in this context
But of that and knoweth no man, no, not the of heaven, but my Father only.
This is of course talking about the second coming. The time of his second coming is also special, sacred knowledge. When the angels don't know, nobody knows! Perhaps, one day they will and perhaps they will also bring this sacred knowledge to prophets. But I expect and am content with never knowing the time before it actually occurs, "quickly" (D&C 39:24) and "like a thief in the night" (1 Thessalonians 5:2).