Few years ago I had the chance to meet this tool, called exiftool, that I used to change the date on several photos that had their dates wrongly set after resetting my old camera.
Now this tool served me again on a bunch of NEF files out of my D7000, that had wrong dates (because I forgot to change the time-zone as soon I returned home from after a long flight from the States), by adding 10Hrs to the dates present on the EXIF fields.
The tool can be downloaded from: *LINK REMOVED*
This tool runs on a command prompt (I use int under Micro$oft Windows, but as far I know it runs under MAC OS X), it is used as a batch command to fix or display the EXIF field of a group of files.
I needed to shift the dates by 10Hrs, therefore I have run the tool as:
(Let's say that the current directory has only the files that had the wrong timestamps)
exiftool -AllDates+=10 *.nef
And all my original NEF files in the current directory had their dates shifted by 10 Hrs. (just as I wanted).
Disclaimer: I do not have any connection to exiftool, Use this tool on your own responsability. I'm just a happy user of it.
Hope it helps.