" So, the WTS has basically derived its creative day doctrine from "Christendom", leading all the way back to some of the so-called Church Fathers. "
Technically not the Church fathers, but the Apostolic fathers, as we are talking of the period around year 80-150, the ones who had known the apostles themselves; the Church fathers belong to the next century(ies). I happen to be reading the apostolic fathers and also early church history just now. The belief has, as others have stated, been abandoned, and especially so after the new and revised edition of "All Scriptures inspired" was published, where it states about Genesis that it refers to the time period starting "in the beginning" in stead of the "47025 BC" or whatever was stated. More nebulous today, yes, but "millions of years" for each day is used.