time_duration.rb

class TimeDuration
  attr_reader :duration

  def initialize(duration, plain=false)
    if plain
      @duration = duration
    else
      init_set_duration(duration)
    end
  end

  def +(time_stamp)
    if time_stamp.is_a? TimeDuration
      TimeDuration.new(time_stamp.duration + @duration, true)
    else
      time_stamp + @duration
    end
  end

  private

  def init_set_duration(duration)
    result = duration.match(/^(\d+)\s+(\w+)$/)
    durations = {
      "day" => 60*60*24,
      "days" => 60*60*24,
      "min" => 60,
      "minute" => 60,
      "minutes" => 60,
      "hour" => 60*60,
      "hours" => 60*60,
      "week" => 60*60*24*7,
      "weeks" => 60*60*24*7,
      "year" => 60*60*24*365,
      "years" => 60*60*24*365
    }
    @duration = result[1].to_i * durations[result[2]]
  end
end

five_minutes = TimeDuration.new("5 min")
five_minutes + Time.now

Time.now

=> 2013-09-13 02:50:06 +0200

five_minutes + Time.now

=> 2013-09-13 02:55:13 +0200