jhead, which is free and in the public domain, will let you do this and many more great things to/from EXIF information. See this hint which also uses jhead and covers installation.
The relevant option to fix timestamps is the -ta option:
-ta(+|-)h[:mm] --> Adjust time by h:mm backwards of forwards. Useful
when having taken pictures with the wrong time set on the camera,
such as when travelling across time zones or DST changes.
Another handy thing you can do with jhead:
-ft --> Set file modification time to Exif time.
[robg adds: I haven't had a reason to use jhead yet, but it's on my hard drive just in case!]

