Speculation has arisen, stating that the figuring of years in the Bible must be according to the “Jewish” or “Prophetic” year. Stemming from the fact that the typical Jewish year is twelve months of exactly 30 days each, the presumption that all years must be figured as 360 days (instead of the actual 365.25 or so years it really is), is often assumed into the text.
This basic hypothesis, however, fails to take into account the basic facts that a year is a year. The “Hebrew Calendar” is not, therefore, 360 day years. Most of the time it is, but, when the calendar is far enough off, based upon the Barley harvest, an additional 30 day month is inserted, to keep the calendar from getting off from the year.
So, the Hebrew year is either 360 days or 390 days, in order to keep the year on track with the actual position of the sun.
As such, we could expect that any calculations of years over a long period of time would use, effectively, the only “year” measure there is, which corresponds to the solar year. Additional months are inserted along the way, so this corresponds to the actual “prophetic year”. Simply calculating the number of days by multiplying 360 by the number of years does produce some interesting results, but it would not be technically accurate, because God created the solar system, and He designed the times and the seasons. The Hebrew calendar takes this into account through the addition of an optional month, and so the calculations would always be based on actual years.
The only exception to this, of course, is the final years, or periods such as three and a half years. When counting by days, a period of three and a half years will always be either 1,260 days, 1275 days (when one extra month is added in the three and a half years), or 1,290 days (when two extra months are in the short-term period).
Attempting to figure long periods of time based on the 360-only rule yields an unrealistic expectation, because the Hebrew Calendar is not always a 360 day year, but is usually 360 days, and sometimes 390.