from js => to "html"
function(element, valueAccessor, allBindings, viewModel, bindingContext) {
      var $element = $(element);
      var observable = valueAccessor();

      if (ko.unwrap(observable)) {
        $element.datepicker('update', observable.datePicker().format('DD.MM.YYYY')); // same as for .datepicker()));
      }
    }