How to sync EXIF data and date information from iPhoto to original files and flickr
ExifTool is a perl script that will give you great power in altering Exif data in images: http://www.sno.phy.queensu.ca/~phil/exiftool/

It's also available through homebrew (http://brew.sh):

$ brew install exiftool

If you are at all okay with working in the terminal it is fairly easy to use (and very powerful if you are dealing with lots of images). There are instructions on the main exifTool page specifically for time/date shifting.