Let's keep some perspective. The vast majority of software applications don't use dates from the 18th century so it's fine to for your app to assume that the Gregorian calendar always was and always will be.
...except that there were countries that weren't using the Gregorian calendar until well into the 20th Century, so you can't even reliably deal with dates as recent as the mid-1900s across borders under that assumption.
Imagine a historical database of Russian birthdays, copied from historical archives by data-entry clerks who had no idea when Russia switched from the Julian to the Gregorian calendar. (For extra fun, imagine that half of the clerks doing the data entry converted the dates before entering them, and half didn’t.)
Software makes assumptions, though most decent time libraries will correctly combine local and date to show the skipped days. It might be a bit lazy though, running "cal sept 1752" will show the same for all locals as far as I can tell from the man page.
