test and possible solution for #3965

gistfile1.diff

diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 433d508..10ae5dc 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -86,7 +86,15 @@ module ActiveRecord
             # be typecast back to 0 (''.to_i => 0)
             value = nil
           else
-            value = column.type_cast(value)
+            # if time zone aware attributes, attribute is 
+            if time_zone_aware_attributes && !self.skip_time_zone_conversion_for_attributes.include?(attr.to_sym) && [:datetime, :timestamp].include?(column.type)
+              unless value.acts_like?(:time)
+                value = value.is_a?(String) ? Time.zone.parse(value) : value.to_time rescue value
+              end
+              value = value.in_time_zone rescue nil if value
+            else
+              value = column.type_cast(value)
+            end
           end
         end
 
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index 4078b7e..1f503ee 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -624,6 +624,20 @@ class AttributeMethodsTest < ActiveRecord::TestCase
       end
     end
   end
+  
+  def test_time_zone_aware_attribute_dirty_tracking
+    time_string_1 = "2012-02-20 10:00:00"
+    time_string_2 = "2012-02-20 09:00:00"
+    
+    in_time_zone 1 do
+      record   = @target.create(:written_on => time_string_1)
+  
+      record.written_on = time_string_2
+      assert record.written_on_changed?, "expected written_on to be changed"
+      record.written_on = time_string_1
+      assert !record.written_on_changed?, "expected written_on not to be changed"
+    end
+  end
 
   def test_setting_time_zone_aware_attribute_to_blank_string_returns_nil
     in_time_zone "Pacific Time (US & Canada)" do