force datetime attributes to be changed, fix GH #3965

activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb

@@ -58,6 +58,7 @@ def #{attr_name}=(original_time)
                 end
                 time = time.in_time_zone rescue nil if time
                 write_attribute(:#{attr_name}, original_time)
+                #{attr_name}_will_change!
                 @attributes_cache["#{attr_name}"] = time
               end
             EOV

activerecord/test/cases/attribute_methods_test.rb

@@ -625,6 +625,19 @@ def test_setting_time_zone_aware_attribute_with_string
     end
   end
 
+  def test_time_zone_aware_attribute_saved
+    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 => '2012-02-20 10:00')
+
+      record.written_on = '2012-02-20 09:00'
+      record.save
+      assert_equal Time.zone.local(2012, 02, 20, 9), record.reload.written_on
+    end
+  end
+
   def test_setting_time_zone_aware_attribute_to_blank_string_returns_nil
     in_time_zone "Pacific Time (US & Canada)" do
       record   = @target.new