I have not cloned the app, but looking at the snapshot I have the feeling that you are confusing the validation of the datepicker with the validation of the textbox.
If you want to validate the datepicker, then you shouldn’t care about what the datepicker gets in input. You should only care about what the datepicker returns, which is automatically validated and constrained by the minimum date.
If you want to validate the textbox, then don’t think about the datepicker. Think about validating a textbox.