I know you can do this in the command-line version of Exiftool, which is a powerful free program, but not at all simple to use. If you're patient and can do the research, you can probably do it here. I think with a search you can come up with the correct commands. But as is often the case, the command line version is finicky about things like spaces, and it can take some practice to get it right. There is an added overlay program called "exiftool gui" which is much easier to use, but I have not found a batch facility in this (though I also have not worked very hard at finding it) . It does have the facility for offsetting the time stamp that you need, but I'm not sure whether you can do batches. Instructions for this program are limited.
Both Exiftool and Exiftool GUI are freeware. To use the GUI version it has to have exiftool in the same folder.
By the way, it's one of the few programs that allows you to change some usually forbidden settings, such as focal length and aperture - I like this because apart from occasionally forgetting to set the "non-cpu" number for a lens, one of my favorites is a preset PC lens which never returns the correct aperture, and some uncoupled macros which return neither focal length nor aperture.
