I have just noticed that Calc seems to use “old dates” correctly. Is this true, and how far back does it go? I have tested a simple “days between two dates” back to 1770’s and it give the correct answer - even when there is a leap year or it rolls over into 1900’s.
I am using Linux Mint and LibreOffice Calc v.5.1.6.2
Not a developer but this an element of the answer.
In Calc, date are encoded in a floating point number starting from an origin called **epoch"", seemingly 1899-12-30 00:00:00.
This number can be signed: positive for dates after the epoch, negative before the epoch. It represents the number of days since the epoch.
Floating-point numbers are IEEE-754 doubles providing a 53-bit mantissa. This allows to store integers up to roughly 10^17, i.e. the date part of the number can be accurate up to this number of days. The fractional part of the number is the fraction of the day which is then converted in HH:MM:SS. However, the larger the number of days, the fewer bits allotted to the time part, meaning when the further away you are from the epoch, the less accurate the time. But you need to be really far fro the epoch to notice a macroscopic effect. Anyway, don’t rely on milliseconds.
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