If the jpg image doesn't have exif header you need to create it first:
jhead -mkexif
Then enter the time stamp data:
jhead -ts2009:08:15-10:30:00
I wanted to have a quick way to rotate the images I take with my digital camera. The Olympus Camedia C-4000 always writes "1" als orientation into the EXIF data (either it doesn't have an orientaiton sensor or it is broken), therefore I cannot rotate the images automatically by the command
jhead -autorot *.jpg
So I created a desktop file ~/bin/rotate90clockwise.desktpop with the following content