The father of Adventism, and hence the Churches of God, was William Miller, an American Baptist preacher active in the 1800s. Know about Willie, know something about the backstory that led to not only Herb Armstrong and the COGs but Seventh-day Adventism and a motley bunch of somewhat related sects which at various stages "cross-pollinated" with the Adventist movement.
Pam Dewey has an excellent discussion on "the Great Disappointment", the inevitable let-down when Willie's prophecies failed dismally, and ties it in with the theory of cognitive dissonance. I found it interesting that Miller wasn't the stereotypical wild-eyed ranter that many assume, and actually moved from a non-controversial Deism to premillennialism over a period of time. Then, as we know, he got caught up in all those confounded numbers in Daniel and Revelation. As do many even unto this very day. Another Willie for example (to wit, William Dankenbring) is currently promoting a new and novel interpretation of the "Seventy Weeks Prophecy" in Daniel chapter 9. Having taking countless pratfalls on prophecy in the past you'd think he would learn to keep his speculations to himself, but alas no. There are lots of dates tossed around in his Prophecy Flash newsletter (affectionately referred to as the Prophecy Flush by some) and long outdated commentaries (Jamieson, Faucett & Brown; Adam Clarke, Alfie Edersheim) are dusted off to back his convoluted logic. You'd need to be a more determined person than me to wade through the schlock in detail but, in the end, Dankenbring determines that 2016 is prophetically significant; fancy that!
But wait, there's more. In the midst of "the week", which the highly knowledgeable Mr. Dankenbring has determined to be a period lasting from 2016 to 2022, the Messiah will return.
Well, it makes about as much sense as 1844 or 1972. The difference is, I guess, that Miller eventually stepped away from prophetic fantasies and number-crunching after he'd made a fool of himself. Our Willie, however, seems ever willing to give it another try.
Maybe he should read Pam's article.