To give a quick usage example, this command added 10 days to each photo:
./jhead -da2007:03:08-2007:02:26 *.JPG
3/8/07 was the date one of the photos was taken, but the data in the EXIF said 2/26/07. This also fixed some photos from the same memory card that said 12/14/06 instead of 12/24/06.
And this command added 10 hours and 2 minutes to each photo. Looks like the camera uses 24 hour time, but when it was set the person didn't realize it. The 2 minutes was to fix the drift it had, conveniently one of the photos had a picture of a digital watch in it.
./jhead -ta+10:02 *.JPG