Hi Philippe,
Using the command-line application this is easy. In the simplest form, the command looks like this:
exiftool -alldates+=1 DIRwhere DIR is the name of a directory containing the images.
For example, I forgot to set the clock on my K-5 back by 1 hour in the fall, so I
used this command.
I think you can probably do this through ExifToolGUI too, but I don't know how.
- Phil