chronic_date_property.rb

module ChronicDateProperty #:nodoc:
  extend ActiveSupport::Concern

  module ClassMethods
    # Define a Reform property which coerces its input via the Chronic parser.
    def chronic_date_property(*args)
      options = args.extract_options!
      field = args.first
      
      property(field, options)

      # Setter (manual coercion)
      define_method("#{field}=") do |val|
        super(Chronic.parse(val).to_date)
      end
    end
  end
end

composite_form_2.rb

class CompositeForm < Reform::Form
  include Composition
  include Reform::Form::Coercion
  include ChronicDateProperty  # our mixin

  model :user

  chronic_date_property :birthdate, on: :profile

  validates :birthdate, presence: true
end

test.rb

test do
  form = CompositeForm.new(user: OpenStruct.new, profile: OpenStruct.new)
  expect(form.validate({ birthdate: "12/03/1985" })).must_equal(true)  # for the sake of completeness

  form.save do |data|
    expect(data[:profile]['birthdate']).must_be_kind_of(Date)  # Now it fails! :(
  end
end