DouKaku?

require 'date'
require 'kconv'
WAREKI = [['明治', '1868/9/8'], ['大正', '1912/7/30'], 
  ['昭和', '1926/12/25'], ['平成', '1989/1/8']]
DATES = %w('1868/12/2' '1926/12/24' '2007/12/01' '1926/12/25'
           '1868/1/2' '1868/100/2')
wareki = WAREKI.sort{|a, b| 
  -1*(Date.parse(a[1]).year <=> Date.parse(b[1]).year)
} # => 新しい年代からマッチさせるためにソート 
DATES.each{|date_str|
  begin
    date = Date.parse(date_str)
    y0, m0, d0 = date.year, date.month, date.day
    era = wareki.each{|nengo, date_str|
      date = Date.parse(date_str)
      y1, m1, d1 = date.year, date.month, date.day
      evaluation = y0 >= y1 && (m0 > m1 or m0 == m1 && d0 >= d1)
      break [nengo, y0 - y1] if evaluation
    }
    puts "#{era[0]}#{era[1] == 0 ? '元' : era[1] + 1}年#{m0}月#{d0}日".tosjis
  rescue
    puts '範囲外'.tosjis
    next
  end
}