WP Code to Time Stamp Last Modified

I need a WP code that allows me to track the modification of documents by placing a time stamp in the document (body or header/footer) that would show the last date and time the document was changed. It should allow formatting the same as the various ones used in WP w/ the date code.

It should only change the date and time stamp when the document has been changed. Just opening the document to view should NOT change the date.

It should be automatic, for example, just as the code for total pages (Tot Pgs Disp) works. If I delete pages or add copy to a document causing the page count to change, it all happens without user interaction. The code simply increments or decrements that total number accordingly.

This date is kept by WP in the document properties, and is also kept in the file properties (at least in Windows) but I have not been able to figure how to get that info into the document on an automatic basis.

Corel/WP touts their chops for the legal industry, but does not provide a means to track simple document iteration. Shame!