As this was the work of many Biblical Scholars, I too would have to look on line as to where the source was. It appears it goes back to AD300 at this link -
1844madesimple - Year-Day Principle
This was the Summary of that link;
:....As the preceding evidence shows, the year-day principle is firmly rooted in Scripture. From the linguistic links between "years" and "days" found in the OT Testament prose and poetry, to clear expressions of it some Old Testament prophecies, the year-day principle can be easily established as a biblical datum.
Meanwhile, from within the prophecies in question themselves, internal evidence points unmistakably to need of the year-day principle.
In short, not only do we have firm proof for the validity of the year-day principle, we showed too why it must be applied to the prophecies in question. Indeed, without the year-day principle, many of those prophecies don't make sense simply because literal time doesn't come anywhere near covering the scope of events depicted in them. In contrast, once the year-day principle is applied, that problem disappears.
This material was condensed and adapted from Selected Studies on Prophetic Interpretation, by William H. Shea. "Year-Day Principle"; Daniel and Revelation Committee Series, Vol. 1. General Conference of Seventh-day Adventists. 1982."
Another site quoting history of the Day for Year -
https://www.ministrymagazine.org/archive/1968/08/day-for-aIt -year-principle
So that is the evidence, we could dismiss it, but when used it Links the Message of Christ, Muhammad, the Bab and Baha'u'llah. What would be the mathematical probability of that happening? After all not many World Messages have been given.
Abdul'baha has said there is no Greater proofs, than these Time Prophecies.
Regards Tony