By default DEVONthink does not allow you to change the creation or modification date of a document. This is intentional as most government agencies and larger companies require that you cannot change the date of a scanned document. If you do not have these requirements and you want to adjust the creation and modification date of documents here is a little script for you.
To install it, unzip the archive and move the .scpt file to ~/Library/Application Support/DEVONthink Pro 2/Scripts. The next time you start DEVONthink Pro or Pro Office the new script will turn up in the Scripts menu. Select any number of documents and choose Scripts > Change Date to show a dialog box that allows you to change the date of the selected items.