NSOpenPanel has a delegate protocol named NSOpenSavePanelDelegate.
And in there, you have the option of saying which files (returned as file URL's) are enabled. The one you'll want is "panel:shouldEnableURL:
", which hands you a URL which you can open (within that delegate method) and look at the various file attributes such as creation or modification dates to decide which ones for which you can return YES ("yes", as in "yes this file can be selected for opening").