app/inputs/datepicker_input.rb
class DatepickerInput < SimpleForm::Inputs::StringInput
def input
input_html_options[:type] = "text"
value = input_html_options[:value]
value ||= object.send(attribute_name) if object.respond_to? attribute_name
input_html_options[:value] ||= I18n.localize(value) if value.present?
input_html_classes << "datepicker"
super # leave StringInput do the real rendering
end
end