Steps to reproduce the issue:
1. Create a Qt Widget project with .ui file generated
2. Edit .ui file in Widget Designer
3. Add a QDateTimeEdit to .ui file
4. Change its timeSpec property

But "timeSpec" should not be there anymore. It's going to be deprecated:
https://doc.qt.io/qt-6/qdatetimeedit.html#timeSpec-prop
Qt Creator also warns about this:

So what has to be done is replacing timeSpec with timeZone. Right now, manually changing the content of "ui_mainwindow.h" helps. But it's better to do it in a more "standard" way - fix the Widget Designer.