○ヶ月○日と表示させる - https://teratail.com/questions/284238#reply-403944

date_duration.rb

require 'active_support'
require 'active_support/core_ext'

#
# 関数の定義
#

def date_duration(base_date, after_date)
  duration_parts =
    ActiveSupport::Duration.build(after_date.to_i - base_date.to_i).parts
  # 起点日も 1日としてカウントする (+1)
  duration_in_days = (after_date - base_date).to_i + 1

  years, months, days, weeks =
    duration_parts[:years], duration_parts[:months], duration_parts[:days], duration_parts[:weeks]

  # * n週 に吸収された値を還元 (n weeks * 7 days)
  # * 起点日も 1日としてカウントする (+1)
  days += weeks * 7 + 1

  case [years, months, days]
  in [0, 0, 0] then "今日"
  in [0, 0, _] then                        "#{days}日後"
  in [0, _, 0] then           "#{months}ヶ月後"    +  "(#{duration_in_days}日)"
  in [0, _, _] then           "#{months}ヶ月#{days}日後(#{duration_in_days}日)"
  in [_, 0, 0] then "#{years}年後"                 +  "(#{duration_in_days}日)"
  in [_, _, 0] then "#{years}年#{months}ヶ月後"    +  "(#{duration_in_days}日)"
  in [_, 0, _] then "#{years}年"    +      "#{days}日後(#{duration_in_days}日)"
  else              "#{years}年#{months}ヶ月#{days}日後(#{duration_in_days}日)"
  end
end

#
# 巻数に渡すデータの定義と関数の使用
#

date_time_args = [
    2020, # year
       7, # month
       3, # day
       0, # hour
       0, # min
       0, # sec
 '+09:00' # time zone
]

base_date  = DateTime.new(*date_time_args)
after_date = base_date.advance(years: 0, months: 1, days: 8)

# For debug
format = '%Y/%m/%d'
pp [base_date, after_date].map { |x| x.strftime(format) }
# => ["2020/07/03", "2020/08/11"]

pp date_duration(base_date, after_date)
# => "1ヶ月9日後(40日)"