Photo meta data editor for OS X?
You can edit Date and Time in iPhoto, and do it in batches too: Photos Menu -> Adjust Date and Time - and it will also correct the date and time on the original file if you choose to.

Another thing to consider: iPhoto catalogues the images on import. If you subsequently change the metadata with a 3rd party App, iPhoto won't recognise this. It will not rescan for changed metadata, so while you're original files may be corrected, it'll make no difference in iPhoto.