This information is stored in the PDF file, it's not the date stamp on the file. This means that when you copy the file (e.g. as an email attachment), the original creation and modification date and time stamps are still intact. The created date is when the PDF file was originally created, the modified date is from the last time you've last modified the file with software that actually honors this part of the meta information and updates it.
Karl Heinz Kremer
PDF Acrobatics Without a Net
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