Dear Slimb,
I emailed her and asked, since I have no easy answer. Here's what I got back:
Dear ---.
Now that’s an interesting question, and I can’t answer it. I don’t know why Literalists accepted the year-day theory. As a principle of prophetic interpretation, it goes back to the 4 th century. Russell knew this. We don’t know where he picked up that tidbit, but we know he came across the fact. Personally, (and it’s just a guess) I think he found it in a book entitled Prophetic History.
Literalists accepted the idea. It was an Adventist who rejected it, though most of them accepted it too. I suppose they accepted it so easily because of the verse in Ezekiel that contains the phrase, “a day shall be for a year.” That gave them license to see it as a God-authorized concept.
I’ll have to research that. Thanks for the interesting, puzzling question.
R