SUMMARY
Entering a date with a 4 digit year is not supported.

STEPS TO REPRODUCE
1. Create a widget with a KDateComboBox widget in Qt designer
2. Test run the widget
3. Open the date picker
4. Enter the year of next year using 4 digits, e.g. 08.08.2022
5. Press return

OBSERVED RESULT
a. The date is shown with 4 digit year info in the KDateComboBox widget in step 2
   (08.08.2021 which is expected for today)
b. The date is represented with 2 digit year info in the line edit of the date picker in step 3
   (08.08.21)
c. Upon pressing return in step 5 nothing happens
d. Entering the date using 2 digit years as 08.08.22 and pressing return closes the date picker as expected but shows 08.08.1922 in the KDateComboBox widget

EXPECTED RESULT
KDatePicker's lineedit allows to enter dates using 4 digits.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE 15.2 stock
(available in About System)
KDE Plasma Version: 5.18.6
KDE Frameworks Version: 5.71.0
Qt Version: 5.12.7

ADDITIONAL INFORMATION
Examples here are based on German date format.
Using the calendar widget of the picker does not show this problem.