As per the question here "Why is God the Father referred to as "Eternal Father" in LDS writings when He is not believed to be eternal?" and in other places, it seems that the LDS church indeed teaches that God the Father had a beginning. When did the church start teaching this? Because for example in their addition to the scripture called the book of Moses, chapter 1 verse 3 states:
" And God spake unto Moses, saying: Behold, I am the Lord God Almighty, and Endless is my name; for I am without beginning of days or end of years; and is not this endless?"
I can't find the reference, but I also seem to recall somewhere in Joseph Smiths other writings where he stated something similar substituting endless for eternal. So when did they start saying that God the Father had a beginning, and have they tried to explain how that is not a contradiction to what their scripture says?