The difference in years between dates

difference_in_years.rb

# Year 2017

module Date
  def self.difference_in_years(left_date, right_date)
    # NOTE: xxxx.yyzz (2017.1231)
    # 
    #       xxxx stands for years
    #       yy   stands for months
    #       zz   stands for days
    # 
    # So that we could move the point on the date scale to determine the major grade (year).
    diff_in_years = (right_date.year - left_date.year)
    diff_in_months = (right_date.month - left_date.month) / 100.0
    diff_in_days = (right_date.day - left_date.day) / 10_000.0

    (diff_in_years + diff_in_months + diff_in_days).truncate
  end
end