I’ll draft a Dubia so that when future popes change the calendar they will include a representative code sample and details about how we handle this.
(It’s important to remember that the pope and friends were operating much more like an international standards body than a religious leader when doing things like this.)
And if there is a contract dated to the 6th October 1582, that does raise questions about its legitimacy (though in general misdating a contract does not void it, it would merely be one point of evidence in your argument that it's a forgery, just like using a particular font in a modern printed contract)
Yeah there are a lot of examples of dates being written two ways on the same document/artifact. In the Anglosphere there are two changes that happened close together: the movement of the new year from Lady Day to the first of January, and the Julian/Gregorian calendar jump, so that confounds things slightly. https://en.wikipedia.org/wiki/Old_Style_and_New_Style_dates