Thanks guys! I have done this:

  def remaining_time
    if expiration != nil
      diff = expiration - Time.now

      days = (diff / 1.day).to_i
      hours = ((diff / 1.hour) % 24).to_i
      mins = ((diff / 1.minute) 60).to_i

      if diff < 7.days
        return days.to_s + "gg, " + hours.to_s + ":" + mins.to_s
      else
        return "more than 7 days"
      end
    end
  end

I'll try to improve it! If you have some advices, they are wellcomes..
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