I posted this question on the Qt forum and received the following reply:
Hi and welcome,
from your post I assume you are just a user of calibre app and no Qt dev, right?!
Can we please have Jun, Jul & Sep accepted as valid input for month names?
The question is:
What widgets do "they" (calibre developers) use there?! Standard widgets or self-developed ones.
That being said, I don't think it's a Qt issue ifself rather than how calibre implemented the calendar logic and defined valid inputs.
Edit:
Found the repo on github:
https://github.com/kovidgoyal/calibre/tree/master
Ok, they are using PySide6 and QCalendar / QCalendarWidget.
I guess they have to change some settings to allow three letter inputs for month names.
(@Steven123
https://forum.qt.io/user/steven123 here would be an example of a Python-based app using Qt frontend)