exiv2 deve ser a solução mais fácil para fazer isso:
exiv2 -O -2 -D -1 -a -12:00:00 adjust /path/to/copy/*.jpg
Trecho da página do manual:
exiv2 - Image metadata manipulation tool
exiv2 [options] [action] file ...
ad | adjust
Adjust Exif timestamps by the given time. Requires at least one of the options -a time, -Y yrs, -O mon or -D day.
-a time
Time adjustment in the format [-]HH[:MM[:SS]]. This option is only used with the 'adjust' action. Examples: 1 adds one hour, 1:01 adds one hour and one minute, -0:00:30 subtracts 30 seconds.
-Y yrs
Time adjustment by a positive or negative number of years, for the 'adjust' action.
-O mon
Time adjustment by a positive or negative number of months, for the 'adjust' action.
-D day
Time adjustment by a positive or negative number of days, for the 'adjust' action.
Além disso, sugiro que tente primeiro em uma cópia das fotos, não nos originais.