Do you just want to put a string into an HTML <input> element labeled “Datum”? or do you want to test the functionality of Date picker GUI?
If you just want to put a string into an HTML <input> element, I suppose you should not bother with the Date picker GUI. See How to handle Date Pickers - #4 by kazurayam for my opinion.
If you want to test Data picker GUI, well, just go on struggling. I suppose it would be hard to accomplish.