Yes, this issue has been well-researched and debated here. You can read the threads below. I believe the Sabbath should be reckoned from midnight to midnight like other days, and that because the Fourth Commandment governs what holy time is set apart for the worship of God, the reckoning of the Sabbath day is not an optional matter but a Fourth Commandment matter.
I didn't realize it was an elders-only forum. Perhaps I should switch.
I guess the reason some people think it is sundown to sundown is because of Jewish practice. The synagogue next door has their sabbath service on Friday evening for that r eason.
I agree it is not optional, that's why I want to figure it out! Thanks for the links.
As I have elsewhere written, I am not persuaded that the Jewish practice of keeping their days from sundown to sundown pre-dates the Babylonian exile.
The Biblical evidence for such an "inverted" measure seems to me to be strongest in the New Testament, whereas an unbaised or non-assuming examination of the OT evidence (as much as I have examined it) leads to my conclusion that originally days were measured from the time of one's "rising"--or from an arbitrary mid-night point, since different people rise at different times.
This is a convoluted way of saying that it seems to me that our duty is to follow whatever the local conventions of telling-time dictate, without raising questions of conscience too high. Be convinced in your own mind.
Is it called the day of preparation in the OT? I know it's called that in the Gospels. It seems to be strongly implied in the account of the gathering of manna.