If by saying gray/invalid days you mean dates before today's date, you can then use the below code. which disallows selecting any date prior to today's date. If not, please put a little code or explain further exactly what you mean.
B4X:
Sub AnotherDatePicker2_DateChanged (NewDate As Long)
If NewDate < DateTime.Now Then
AnotherDatePicker2.Date = DateTime.Now
End If
End Sub
There is also B4XDateTemplate in XUI Views you can use
I'm not familiar with ADP, bur maybe there is a setting to alter the behaviour you/I see. No doubt looking at the linked examples and comparing with your usage will reveal all.