[MrManatee]

I wouldn't say it makes "little sense". Just like we can talk about the year 776 BC even though no one at the time called it that, we can extend the Gregorian calendar backwards to dates when it wasn't used anywhere. The Wikipedia article on the proleptic Gregorian calendar lists some use cases. [1]

And in any case, 15 October 1582 isn't some hard cutoff point where we can stop worrying about calendar conversions. Only four countries adopted the Gregorian calendar on that day, and even in Europe there are several countries that only switched in the 20th century. If a piece of software needs to support historical dates that go anywhere near 1582, it needs to be aware of the existence of different calendars.

[1] https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar