Determine the number of months between two time stamps

months_between.rb

# A direct calculation of the number of months between two dates

class Time

  class << self

    def months_between(start_date, end_date)
      return -months_between(end_date, start_date) if end_date < start_date

      s = start_date
      t = end_date

      whole_months = (t.year * 12 + t.month) - (s.year * 12 + s.month)

      # handle variable length months
      s_last_day = days_in_month(s.month, s.year)
      t_last_day = days_in_month(t.month, t.year)

      if s.day == s_last_day && t.day == t_last_day && s.day >= t.day
      elsif s.day == s_last_day && t.day == t_last_day && s.day > t.day
        return whole_months
      elsif s.day == s_last_day && s.day > t.day
        return whole_months - 1
      elsif s.day == s_last_day && s.day == t.day
      elsif s.day == s_last_day && s.day < t.day
        return whole_months
      elsif t.day == t_last_day && s.day >= t.day
      elsif t.day == t_last_day && s.day < t.day
        return whole_months
      elsif s.day > t.day
        return whole_months - 1
      elsif s.day == t.day
      elsif s.day < t.day
        return whole_months
      end

      # handle time part
      [:hour, :min, :sec].each do |part|
        if s.send(part) > t.send(part)
          return whole_months - 1
        elsif s.send(part) <= t.send(part)
          return whole_months
        end
      end

      whole_months
    end

  end

end

months_between2.rb

# This is a very simple yet potentially inefficient way of determining the months between two dates
# The method takes longer the farther the start and end dates are from each other

class Time

  class << self

    def months_between2(start_date, end_date)
      return -months_between2(end_date, start_date) if end_date < start_date

      count = 1
      while true
        return count - 1 if (start_date + count.months) > end_date
        count += 1
      end
    end

  end

end

months_between_test.rb

# A method with lots of test cases comparing the two methods of calculating the number of months between two dates
# The test runs both methods of calculation against the two given dates and compares them.  Outputs when there are differences.

def self.test_months

  # a set of start and end date
  start_dates = [ '2011-02-12', '2011-02-27', '2011-02-28', '2011-03-13', '2011-03-30', '2011-03-31',
                  '2011-04-29', '2011-03-30', '2011-05-30', '2011-05-31' ]
  end_dates   = [ '2011-02-12', '2011-02-14', '2011-04-25', '2011-04-30', '2011-05-17', '2011-05-30',
                  '2011-05-31', '2011-07-25', '2011-07-30', '2011-07-31', '2011-08-05', '2012-02-28',
                  '2012-02-29' ]

  # a set of times fo append to above dates
  time_stamps = [ '00:00:00', '00:00:00', '00:00:00', '01:56:23', '04:23:42', '12:04:34', '16:32:03', '22:22:22' ]

  # a set of start and end dates that proved useful in finding edge cases
  specials = [ [ '2011-03-31 16:32:03', '2011-07-31 12:04:34' ],
               [ '2011-03-31 22:22:22', '2012-02-29 12:04:34' ],
               [ '2011-03-30 22:22:22', '2012-02-29 01:56:23' ],
               [ '2011-05-31 12:04:34', '2011-07-31 00:00:00' ],
               [ '2011-02-28 01:56:23', '2012-02-29 00:00:00' ],
               [ '2011-03-30 22:22:22', '2012-02-29 00:00:00' ],
               [ '2011-02-28 04:23:42', '2011-07-31 00:00:00' ],
               [ '2011-02-28 16:32:03', '2011-04-30 00:00:00' ],
               [ '2011-02-28 16:32:03', '2011-07-31 12:04:34' ],
               [ '2011-02-28 16:32:03', '2012-02-28 00:00:00' ],
               [ '2011-03-30 22:22:22', '2012-02-29 04:23:42' ],
               [ '2011-03-31 16:32:03', '2011-04-30 04:23:42' ],
               [ '2011-05-31 04:23:42', '2012-02-29 01:56:23' ],
               [ '2011-03-31 12:04:34', '2011-04-30 04:23:42' ],
               [ '2011-03-31 01:56:23', '2011-04-30 00:00:00' ],
               [ '2011-03-31 04:23:42', '2012-02-29 00:00:00' ],
               [ '2011-05-31 16:32:03', '2012-02-29 01:56:23' ]
            ]

  # A set of test cases that cover all combinations of:
  # start_date is last day of the month
  # end_date is last day of the month
  # start_date.day <=> end_date.day
  # start_date.day == end_date.day && start_date.day <=> end_date.day
  special2 = [ [ '2011-03-31 00:00:00', '2011-04-30 00:00:00' ], # 1
               [ '2011-03-31 03:12:43', '2011-05-31 13:32:54' ], # 2
               [ '2011-03-31 06:17:23', '2011-05-31 06:17:23' ], # 3
               [ '2011-03-31 00:00:00', '2011-05-31 00:00:00' ], # 4
               [ '2011-04-30 00:00:00', '2011-05-31 00:00:00' ], # 5
               [ '2011-03-31 00:00:00', '2011-06-27 00:00:00' ], # 6
               [ '2011-02-28 03:12:43', '2011-07-28 13:32:54' ], # 7
               [ '2011-02-28 06:17:23', '2011-07-28 06:17:23' ], # 8
               [ '2011-02-28 13:32:54', '2011-07-28 03:12:43' ], # 9
               [ '2011-02-28 00:00:00', '2011-05-29 00:00:00' ], # 10
               [ '2011-03-30 00:00:00', '2012-02-29 00:00:00' ], # 11
               [ '2011-04-30 03:12:43', '2011-06-30 13:32:54' ], # 12
               [ '2011-04-30 06:17:23', '2011-06-30 06:17:23' ], # 13
               [ '2011-04-30 13:32:54', '2011-06-30 03:12:43' ], # 14
               [ '2011-04-30 00:00:00', '2011-07-31 00:00:00' ], # 15
               [ '2011-03-17 00:00:00', '2011-07-15 00:00:00' ], # 16
               [ '2011-03-17 03:12:43', '2011-08-17 13:32:54' ], # 17
               [ '2011-03-17 06:17:23', '2011-08-17 06:17:23' ], # 18
               [ '2011-01-17 13:32:54', '2011-07-12 03:12:43' ], # 19
               [ '2011-01-17 00:00:00', '2011-07-12 00:00:00' ], # 20
               [ '2011-03-12 00:00:00', '2011-09-23 00:00:00' ], # 21
             ]

  total_right = 0
  total_wrong = 0

  start_dates.each do |s|
    end_dates.each do |e|
      st = Time.parse("#{s} #{time_stamps[rand(time_stamps.length)]}")
      et = Time.parse("#{e} #{time_stamps[rand(time_stamps.length)]}")
      m1 = Time.months_between(st, et)
      m2 = Time.months_between2(st, et)
      if m1 != m2
        puts "#{st} <=> #{et} :: #{m1} <=> #{m2}"
        total_wrong += 1
      else
        total_right += 1
      end
    end
  end

  puts "-------------------------------"

  (specials + special2).each do |s|
    st = Time.parse(s[0])
    et = Time.parse(s[1])
    m1 = Time.months_between(st, et)
    m2 = Time.months_between2(st, et)
    if m1 != m2
      puts "#{st} <=> #{et} :: #{m1} <=> #{m2}"
      total_wrong += 1
    else
      total_right += 1
    end
  end

  [total_right, total_wrong]
end