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.

1 files changed, 18 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/core_ext/date/calculations.rb b/activesupport/lib/active_support/core_ext/date/calculations.rb
index fef49e1003..2612dca93a 100644
--- a/activesupport/lib/active_support/core_ext/date/calculations.rb
+++ b/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