as long as a user selects the date from the calendar, the add-on works fine

Directory Enhanced DatePicker
1

as long as a user selects the date from the calendar, the add-on works fine
but if a user enters the date directly into the input field then letters can also be entered
can that be prevented somehow?
e.g. with the help of the TextField Formatter add-on

2

The input is not accepted as you see the field will turn red as signal of invalid input. But user can type invalid input to field, yes. This behavior is intentional.