Backports Date#>> from 1.9 so that calculations do the right thing around the calendar reform

Our next_month gives November for some late dates in September of 1582.
Related methods, last_*, and in general advance have the same issues.
This commit fixes those, see the test suite for expected behavior, which
we still run in 1.9 to ensure it matches as we do with other methods
defined in Date in 1.9.

activesupport/CHANGELOG

@@ -1,5 +1,7 @@
 *Rails 3.0.0 [beta 4/release candidate] (unreleased)*
 
+* Backports Date#>> from 1.9 so that calculations do the right thing around the calendar reform. [fxn]
+
 * Date#to_time handles properly years in the range 0..138. [fxn]
 
 * Deprecate {{}} as interpolation syntax for I18n in favor of %{} [José Valim]

activesupport/lib/active_support/core_ext/date/calculations.rb

@@ -4,6 +4,24 @@ require 'active_support/core_ext/time/zones'
 require 'active_support/core_ext/object/acts_like'
 
 class Date
+  if RUBY_VERSION < '1.9'
+    undef :>>
+    
+    # Backported from 1.9. The one in 1.8 leads to incorrect next_month and
+    # friends for dates where the calendar reform is involved. It additionally
+    # prevents an infinite loop fixed in r27013.
+    def >>(n)
+      y, m = (year * 12 + (mon - 1) + n).divmod(12)
+      m,   = (m + 1)                    .divmod(1)
+      d = mday
+      until jd2 = self.class.valid_civil?(y, m, d, start)
+        d -= 1
+        raise ArgumentError, 'invalid date' unless d > 0
+      end
+      self + (jd2 - jd)
+    end
+  end
+
   class << self
     # Returns a new Date representing the date 1 day ago (i.e. yesterday's date).
     def yesterday

activesupport/test/core_ext/date_ext_test.rb

@@ -145,6 +145,10 @@ class DateExtCalculationsTest < Test::Unit::TestCase
     assert_equal Date.new(1999,2,28), Date.new(2000,2,29).last_year
   end
 
+  def test_last_year_in_calendar_reform
+    assert_equal Date.new(1582,10,4), Date.new(1583,10,14).last_year
+  end
+
   def test_next_year
     assert_equal Date.new(2006,6,5), Date.new(2005,6,5).next_year
   end
@@ -153,16 +157,28 @@ class DateExtCalculationsTest < Test::Unit::TestCase
     assert_equal Date.new(2001,2,28), Date.new(2000,2,29).next_year
   end
 
+  def test_next_year_in_calendar_reform
+    assert_equal Date.new(1582,10,4), Date.new(1581,10,10).next_year
+  end
+
   def test_yesterday
     assert_equal Date.new(2005,2,21), Date.new(2005,2,22).yesterday
     assert_equal Date.new(2005,2,28), Date.new(2005,3,2).yesterday.yesterday
   end
 
+  def test_yesterday_in_calendar_reform
+    assert_equal Date.new(1582,10,4), Date.new(1582,10,15).yesterday
+  end
+
   def test_tomorrow
     assert_equal Date.new(2005,2,23), Date.new(2005,2,22).tomorrow
     assert_equal Date.new(2005,3,2),  Date.new(2005,2,28).tomorrow.tomorrow
   end
 
+  def test_tomorrow_in_calendar_reform
+    assert_equal Date.new(1582,10,15), Date.new(1582,10,4).tomorrow
+  end
+
   def test_advance
     assert_equal Date.new(2006,2,28), Date.new(2005,2,28).advance(:years => 1)
     assert_equal Date.new(2005,6,28), Date.new(2005,2,28).advance(:months => 4)
@@ -174,6 +190,17 @@ class DateExtCalculationsTest < Test::Unit::TestCase
     assert_equal Date.new(2005,2,28), Date.new(2004,2,29).advance(:years => 1) #leap day plus one year
   end
 
+  def test_advance_calendar_reform
+    assert_equal Date.new(1582,10,15), Date.new(1582,10,4).advance(:days => 1)
+    assert_equal Date.new(1582,10,4), Date.new(1582,10,15).advance(:days => -1)
+    5.upto(14) do |day|
+      assert_equal Date.new(1582,10,4), Date.new(1582,9,day).advance(:months => 1)
+      assert_equal Date.new(1582,10,4), Date.new(1582,11,day).advance(:months => -1)
+      assert_equal Date.new(1582,10,4), Date.new(1581,10,day).advance(:years => 1)
+      assert_equal Date.new(1582,10,4), Date.new(1583,10,day).advance(:years => -1)
+    end
+  end
+
   def test_next_week
     assert_equal Date.new(2005,2,28), Date.new(2005,2,22).next_week
     assert_equal Date.new(2005,3,4), Date.new(2005,2,22).next_week(:friday)