some_model.rb

class SomeModel < ActiveRecord::Base

  include TemporalHelper

  def my_temporal_attribute=(value)
    write_attribute :my_temporal_attribute, parse_temporal(value)
  end

end

temporal_helper.rb

module TemporalHelper

  def parse_temporal(temporal)
    case temporal
    when NilClass, ''
      nil
    when Time, Date, DateTime
      temporal
    else
      temporal = temporal.to_s
      Chronic.parse(temporal) || Time.parse(temporal)
    end
  end

end