date_field helper for HTML5 type="date" inputs! And a version of value_before_type_cast that returns a string in ISO-8601 format as the value for Dates, as specified by http://www.whatwg.org/specs/web-apps/current-work/multipage/states-of-the-type-attribute.html#date-state-(type=date)

form_helper_extensions.rb

# Extensions to gems/actionpack-3.2.13/lib/action_view/helpers/form_helper.rb

=begin
A version of InstanceTag.value_before_type_cast that returns a string in ISO-8601 format for Date columns.

The original behavior of value_before_type_cast was to return the Date object itself. But the default to_s on a Date returns a string like "June 5, 2013".
    
However, HTML5 date input fields expect their values to be in ISO-8601 format ("2013-06-05") and won't recognize the value if it is in some other format.

This makes it so you can simply do this:
  f.date_field :some_date
and have it work as expected.

Otherwise you'd have to do something like this for every date_field:
  f.date_field :some_date, value: f.object.some_date.to_s(:db)
=end

module ActionView
  module Helpers
    module FormHelper
      InstanceTag.class_eval do
        class << self
          def value_before_type_cast_with_date_to_s(*args)
            value_before_type_cast_without_date_to_s(*args).tap do |_|
              return _.to_s(:db)  if _.is_a? Date
            end
          end
          alias_method_chain :value_before_type_cast, :date_to_s  unless method_defined? :value_before_type_cast_without_date_to_s
        end
      end

      # HTML5 date field. This should be part of Rails, like phone_field, email_field!
      def date_field(object_name, method, options = {})
        InstanceTag.new(object_name, method, self, options.delete(:object)).to_input_field_tag("date", options)
      end
    end
  end
end

module ActionView
  module Helpers
    FormBuilder.class_eval do
      %w(date_field).each do |selector|
        class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
          def #{selector}(method, options = {})  # def text_field(method, options = {})
            @template.send(                      #   @template.send(
              #{selector.inspect},               #     "text_field",
              @object_name,                      #     @object_name,
              method,                            #     method,
              objectify_options(options))        #     objectify_options(options))
          end                                    # end
        RUBY_EVAL
      end
    end
  end
end