This are more like a theoretical series of questions, not practical ones.
Why is not the zero date in Calc (or Excel) the very last day of the previous century? If 20th century ended on December 31st, 1900, this should be date zero, so in number format in Calc, date 1 ought to be January 1, 1901, the first day of the first month of the first year.
I see it even worse: zero date is not the last day of December, it is December 30. That doesn’t make sense to me. The first day is last day of previous year!
Also I assume (possible wrong) that year 1899 was chosen on the belief that 1899 was the last year of the century, which is not correct. If there are historical, anecdotal reasons, I would love to understand them.
Apart, is there a chance that I assume/set a zero date according to what I just wrote above? For many date calculations, that would make my life easier in a accounting/clerk sense.