Supports only Meiji dates and later. But it works!

calendar.rb

@@ -0,0 +1,19 @@
+#encoding: utf-8
+module Calendar
+  def self.convert(date)
+    return nil unless date
+    year, month, day = date.split('-').map(&:to_i)
+    if Date.parse(date) >= Date.parse("1989-01-08")
+      japanese_year = "平成#{(year - 1989 + 1).to_s.rjust(2, '0')}"
+    elsif Date.parse(date) >= Date.parse("1926-12-25")
+      japanese_year = "昭和#{(year - 1926 + 1).to_s.rjust(2, '0')}"
+    elsif Date.parse(date) >= Date.parse("1912-07-30")
+      japanese_year = "大正#{(year - 1912 + 1).to_s.rjust(2, '0')}"
+    elsif Date.parse(date) >= Date.parse("1868-09-08")
+      japanese_year = "明治#{(year - 1868 + 1).to_s.rjust(2, '0')}"
+    else
+      raise ArgumentError, "Dates earlier than the Meiji era are unsupported"
+    end
+    "#{japanese_year}年#{month.to_s.rjust(2, '0')}月#{day.to_s.rjust(2, '0')}日"
+  end
+end

calendar_spec.rb

@@ -0,0 +1,38 @@
+# encoding: utf-8
+
+require 'rubygems'
+require 'rspec'
+require "#{File.dirname(__FILE__)}/calendar"
+
+describe Calendar do
+  it "should return a blank string for nil input" do
+    Calendar.convert(nil).should == nil
+  end
+
+  it "should work for today" do
+    Calendar.convert("2012-10-20").should == "平成24年10月20日"
+  end
+
+  it "should work for border of era" do
+    Calendar.convert("1989-01-07").should == "昭和64年01月07日"
+    Calendar.convert("1989-01-08").should == "平成01年01月08日"
+    Calendar.convert("1989-01-09").should == "平成01年01月09日"
+  end
+
+  it "should work for Showa dates (natsukashii)" do
+    Calendar.convert("1979-08-08").should == "昭和54年08月08日"
+  end
+
+  it "should work for Taisho dates (furui)" do
+    Calendar.convert("1912-07-30").should == "大正01年07月30日"
+  end
+
+  it "should work for Meiji dates (motto furui)" do
+    Calendar.convert("1868-09-08").should == "明治01年09月08日"
+  end
+
+  it "should throw error for pre-Meiji dates" do
+    expect{Calendar.convert("1868-09-01")}.to raise_error(ArgumentError)
+  end
+end
+