membership.rb

class Membership < ActiveRecord::Base
  value_object :period, Period  # using: https://github.com/kasperbn/value_object_ar
end

period.rb

class Period

  attr_accessor :value

  def initialize(value)
    @value = value
  end

  def end_date
    (value.last == Float::INFINITY) ? value.last : (value.last - 1.day)  # PostgreSQL daterange values get loaded by Rails with an exclusive upper bound
  end

  def start_date
    value.first
  end

end