>I check with some expert in the office and you can check some environment variables in AutoCAD to know the Creation date, the LastUpdate date and the elapse time from last modification.
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>This means that you need to create code to control this. We create code that is run when the user click on save in AutoCAD. But this code is not public domain. :(

If the code is for sale or if you can provide the service for a few file, my client may be very interested.