formtastic_date.rb

module ActionView::Helpers::FormTagHelper
  def date_field(object_name, method, options = {})
    ActionView::Helpers::InstanceTag.new(object_name, method, self, options.delete(:object)).to_input_field_tag("date", options.merge(:size => nil))
  end
end

class ActionView::Helpers::FormBuilder
  def date_field(method, options = {})
    @template.send(
      "date_field",
      @object_name,
      method,
      objectify_options(options))
  end
end

class Formtastic::SemanticFormBuilder
  def date_input(method, options)
    basic_input_helper(:date_field, :string, method, options)
  end
end