Rename from file name: mm-dd-yy TO yyyy-mm-dd

Years ago, I saved many images named in this method:
mm-dd-yy
09-21-12.jpg

I have no exif data to work with and am simply wanting to rename hundreds of images based on their existing file name, so that they are consistent with the other thousands named as
yyyy-mm-dd
2012-09-22

Renaming:

FROM > TO
mm-dd-yy > yyyy-mm-dd
09-21-12. > 2012-09-22

Thanks for any and all help with this.
Josh