SyncFF is a utility app and service for setting the modification and creation date of folders to the most recent modification date of their contained files (and, optionally, folders.) The process is recursive, so all subfolders are processed too.
To use SyncFF just drop a folder on it or the main status window. Alternatively you can use the File menu item "Select Folder To Sync…" or the service menu items "Sync Folder's Dates To Last Modified File."
The main status window displays progress of processing.