Here’s an excellent article, EXACTLY on point of our discussion. It is NOT written by a SDA, but it does quote Leroy Froom (SDA) and his supporting article.
The BIBLICAL PROPHETIC YEAR ITS LENGTH, ORIGIN AND APPLICATION
ZIONS TOWER of the MORNING TRACT PUBLICATIONS P. O. Box 28021 Detroit, Mich. 48228 1983
From the Conclusion:
In approaching the biblical prophetic year, the first task was to define its length, as determined by a comparison of related Scriptures containing similar time expressions. By subjecting these to a formal mathematical reasoning process, it was found that the prophetic year was equivalent to 360 days, and the prophetic month to 30 days.
The next inquiry, almost a parenthesis to the study, was to probe the origin of these lengths; that is, to consider how the Creator might have decided upon employing a 360-day year and a 30-day month for use in biblical prophecy. The conclusions, of course, could only be suggestive, since they are not positively stated in the Bible. Nonetheless, in correlating Scripture with naturally recurring motions of the sun, earth, and moon, certain relationships were evident. An average of the solar and “lunar” years yielded 359.80 days, and of the Gregorian and Jewish months, 29.98 days, almost identical with the prophetic 360-day year and 30-day month.