>> value = Marshal.dump comment
=> "\004\bo:\fComment\a:\020@attributes{\n\"\017updated_at\"\0302008-08-25 14:45:49\"\ttext\"\017Nice post.\"\fpost_id\"\0061\"\aid\"\016865423346\"\017created_at\"\0302008-08-25 14:45:49:\026@attributes_cache{\000"
>> Marshal.load value
=> #<Comment id: 865423346, post_id: 1, text: "Nice post.", created_at: "2008-08-25 14:45:49", updated_at: "2008-08-25 14:45:49">
>> comment.update_attributes :text => 'Nice post!'
=> true
>> value = Marshal.dump comment
=> "\004\bo:\fComment\t:\030@changed_attributes{\000:\020@attributes{\n\"\017updated_atu:\tTime\r.\037\e\200\241?վ\"\ttext\"\017Nice post!\"\fpost_id\"\0061\"\aid\"\016865423346\"\017created_at\"\0302008-08-25 14:45:49:\026@attributes_cache{\000:\f@errorso:\031ActiveRecord::Errors\a;\n{\000:\n@base@\000"
>> Marshal.load value
=> #<Comment id: 865423346, post_id: 1, text: "Nice post!", created_at: "2008-08-25 14:45:49", updated_at: "2008-08-25 16:47:45">

# look at the first and last updated_at values