How might the math have come out if you had used a 360 day "prophetic calendar" for the Old Testament prophecy?
http://www.beholdthebeast.com/mathematical_precision_of_prophecy.htm#daniel_1290_days
From the article we read:-
Old Testament - day = year "language" is suggested in Genesis' account of the flood as 30 day months pointing to a 360 day prophetic year, each year being equal to .9857 solar year (360/365.24) of or our modern historical record that the event dating comes from. 1290 x .9857 = 1271.5 solar years. Subtracting 583 we arrive again at 688.5 AD.
I'm not very convinced that this methodology is appropriate. Reason being that both the Jewish "prophetic" 360 day calendar and our own modern 365 day calendar have regular adjustments built in, to keep then in sync with the seasons. So over a very long period of say 2500 years I would expect them not to vary significantly in terms of the number of days elapsed.
In the Jewish 360 day calendar, an extra 30 day month is added to every sixth year.
In our modern 365 day calendar, an extra day is added to February every fourth year.
So I did a quick calculation of how many days would have elapsed during 2500 Jewish years:-
Normal days = 2500 x 360 = 900,000 days
Number of adjustments = 2500 / 6 = 416.67
Number of adjustment days = 416.67 x 30 = 12,500 days
TOTAL TIME ELAPSED = 900,000 + 12,500 =
912,500 daysSimilar calculation for our modern 365 day calendar:-
Normal days = 2500 x 365 = 912,500 days
Number of adjustments = 2500 / 4 = 625
Number of adjustment days = 625 x 1 = 625 days
TOTAL TIME ELAPSED = 912,500 + 625 =
913,125 daysSo not much of a difference for such a long period of time, ie 2500 years. And not being an expert of the Jewish calendar, there maybe other adjustments which I haven't taken account of, to close the gap even more.
Anyway, using this as a guide, the factor to apply to Jewish "prophetic" years would work out as follows:-
912,500 / 913,125 = 0.999316
So the 1,290 day/years of Daniel 12:11 would work out as 1290 x 0.999316 = 1289.11764 years, which doesn't really work too well.
So given the fact that both calendars aim to stay in sync with the seasons, via regular adjustments, it's probably better not to make any adjustment when dealing with large periods of time expressed as years. Ie, 2500 Jewish years would be pretty much equivalent to 2500 modern years, in terms of overall time elapsed in days.