def years_old
      return nil if present_day < birthday.date
      format = "%Y%m%d"
      (present_day.strftime(format).to_i - birthday.date.strftime(format).to_i) / 10000
    end
def years_old
      return nil if present_day < birthday.date
      format = "%Y%m%d"
      (present_day.strftime(format).to_i - birthday.date.strftime(format).to_i) / 10000
    end
Happybirthday::Age initializer
 @param birthday [Happybirthday::Birtyday] birthday object
 @param present_day [Date] a day for calculating age
 @return [Happybirthday::Age]
 Get age
 @return [Integer] if Age.present_day is after birthday
 @return [nil] if Age.present_day is before birthday