My understanding of the process is that you will have to save the attachment to the hard drive in order to read the creation date, which will thus always be newer than the last saved version as the creation date will be the date you save it from attachment to folder. For this reason I would advocate saving all attachments with unique names to avoid the issue of overwriting required data. If you process the messages as they arrive, using a rule, then surely you should also have the latest version of the file? You will have to modify your code to use it as a script for use with a rule.
You can use the FileNameUnique function at
http://www.gmayor.com/useful_vba_functions.htm to provide unique names for the attachments.