my friend,, I tried your code.
if you changed self.dateEdit.setDate(QDate(2021, 3, 1)) to self.dateEdit.setDate(QDate(2021, 1, 1))
and tried to type 2021-03-26 , you will get a problem (bug)
I can't set date to 2021-03-01, because when you design an application, you will set the date to the default which is year/01/01 .