Why doesn't it seem as crazy when it was done 150+ years ago? William Miller did the same thing. He grabbed random passages of Scripture and tied them together to predict when Jesus was coming back. We've all heard the one Adventists use, but here it is again:
I can prove it by the length of the vision which Daniel had, (viii. 1-14,) of the ram, he‑goat, and little horn, which Daniel was informed was 2300 days long. Dan. viii. 13,14; "Then I heard one saint speaking, and another saint said unto that certain saint which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot? And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed." And from which the 70 weeks were cut off, and fulfilled, the year of Christ's death. Dan. ix. 20—27. Then 70 weeks be, years cut off from 2300 days, makes these days years; and 490 years fulfillled in A, D. 33, leaves 1810 years to the fulfillment of the vision, which added to 33 makes 1843, when the sanctuary will be cleansed and the people of God justified.We've likely heard this multiple times, and while we don't necessarily understand it (that's a lot of numbers!), it sounds like it could make sense. But did you know that William Miller had 15 ways to "prove" 1843 (later, 1844)? Why doesn't the Adventist Church use all 15? It would certainly make their case stronger to have 15 proofs as opposed to 1, right? The answer is obvious when you read through them. Miller, though likely well-intentioned, was grasping at straws. He was taking things all over the Bible, piecing them together, and coming up with 1843. He was convinced that Jesus was returning then, so he was adding up random numbers to prove it.
Here is just one example, but if you want to read all of them, check out the link at the bottom of the page.
It can also be proved by the words of Christ, Luke xiii.32: "And he said unto them, Go ye, and tell that fox, Behold, I cast out devils, and I do cures to day and to morrow, and the third day I shall be perfected." These two days, in which Christ casts out devils and does cures, are the same as Hosea's two days, at the end of which, the devil will be chained, and cast out of the earth into the pit, and shut up. This will take 2000 years of the roman power. Rev. xii. 9: "And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him." And then the people of God will be perfected. Rev. xx. 6: " Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years."Make sense to you? Me either! And there are 13 more just as twisted. William Miller later admitted his mistakes, nevertheless, from him mistakes began the SDA Church.
This time began with the "great dragon," Rev. xii. 3,4: "And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born."
This government will draw after him one third part of the time, which wicked men have power in the earth, viz., 6000 years; and the 7000th, the Christ will take possession and reign with his saints, in perfect bless.
This dragon power began its power over the saints when the league was made with him, B. C. 158,—and will end in 1842. Then the third day will begin 1843. (See Sec. Ad. Lib. No. 3, page 61,)
"We expected the personal coming of Christ at that time; and now to contend that we were not mistaken, is dishonest. We should never be ashamed frankly to confess our errors. I have no confidence in any of the new theories that grew out of that movement, namely, that Christ then came as the Bridegroom, that the door of mercy was closed, that there is no salvation for sinners, that the seventh trumpet then sounded, or that it was a fulfillment of prophecy in any sense." History of the Advent Message, p. 410, 412.Regardless of the fact that Miller confessed his mistakes, Ellen White and Adventism adopted 1 of his 15 proof methods. They didn't use it in building a small doctrine, but it became the very pillar of the church, according to Ellen White.
"The scripture which above all others had been both the foundation and central pillar of the Advent faith was the declaration, 'Unto two thousand and three hundred days; then shall the sanctuary be cleansed.'" 4SP 258.1http://www.truthorfables.com/Miller's_Time%20_Proved_15_Ways.htm - Miller's 15 proofs
http://www.ellenwhiteexposed.com/2300.htm - more studies on the 2300 day prophecy