When you crawl content from a file share, you’ll notice that when SharePoint displays search results, all the displayed dates are exactly the same. Instead of displaying the date the file was last modified, it seems to display the date on which the file was added to the search index.
When you want to display the date last modified (which should be default imho, as it was in 2007), you need to edit the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0\Search\Global\Gathering Manager
Set the value to 0 (zero) instead of 1.
After this, start a full crawl to refresh the index. Now search again and you’ll notice the date last modified is shown.
This only goes for local file share content sources, the default SharePoint content source doesn’t have this problem.
Special thanks to Dan Arr who provided this solution on MSDN forums: http://social.technet.microsoft.com/Forums/en-US/sharepoint2010setup/thread/8a72c00d-f659-44d5-98ef-e3e52c44a6f7