Question

The exact halakhic date line is a matter of many opinions.
I understood that it is agreed that the U.S. is several hours behind us (about 7 hours), and not 17 hours ahead of us.
Mainly because the spread of human beings was by ships from Europe to the U.S., and as they advanced on their voyages the day lengthened with them a little each day, so that the day on which their feet came to rest on American soil was a continuation of the day in the settled world of humanity at the time — that is, Europe, Asia, and Africa.
They dragged the days along with them.
I learned a bit more and understood that already about 14,000 years ago people simply walked from Russia to the U.S. and reached the American continent from the east, and from there spread out. (Later there was warming, the water level rose, and the way back to Europe was blocked off from them, and since then it has been by voyages or flights, mainly from Europe.)
If that is true,
and one goes by the way human beings spread then (before the giving of the Torah), then the Sabbath in the U.S. is at the sunset of a day before us, and not at the sunset of the Sabbath of Europe only a few hours later.
Or do we specifically go by Columbus's voyage, of blessed memory, since that was 500 years ago and not about 14,000 years ago, or because it is documented and the earlier one is not?
Or because that was before the giving of the Torah and this was after?
Or because Columbus, of blessed memory, was a Jewish forced convert, whereas those from 14,000 years ago were just some ugly Homo sapiens?
Or because Jewish law does not really recognize what happened more than 5,700 years ago?
Or perhaps at any rate, because of a Torah-level doubt, our brothers in the U.S. would have to observe two days every Sabbath out of doubt?
 
What is the Rabbi's view on this matter?