After switching time zones I always seem to have the problem of forgetting to update the internal clock of my cameras which screws up the exif-data in the pictures. But I found the program exiv2 (available in Ubuntu’s repositories) that has a very simple syntax for manipulating exif-data.
A simple
exiv2 -a -6 adjust *
turns the clock back 6 hours of all pictures in the current folder. Full documentation at http://linux.die.net/man/1/exiv2