Not sure. My intuition is that photos are completely read-only; I don't think it will ever change them, so tags might be stored in some other opaque storage.

I do know that Carousel uses the DateTimeOriginal EXIF tag to set the date. You can use this command to re-tag photos, for example:

    exiftool -DateTimeOriginal="2009:01:01 00:00:06" DSCF0038.JPG