I am wondering: do you really have to pick the date from the popup ?
I see that there is a field next to the calendar button.
A) Try to type into that field the whole date in the expected format (dd/MM/yyyy maybe ?)
B) If the field is not editable, then we will use the technique of get and set attribute to enforce the content of the field To do so, we need you to open UiExplorer and send us a screenshot of what the UiExplorer gives, so we help you chose the attributes to enforce in the field: