I was about to suggest something...then realized you are asking about incrementally changing the timestamp, which is in the EXIF data of the pics, not the filename . Did a little digging, and found a bunch of for-sale programs that look nice and polished, but then stumbled across a freeware program called Exifer:
Exifer
The author states that he stopped working on it 5 years ago, but I just tried it out and although its clunky it does work to batch-change the timestamps with increments of time that you choose. I also discovered that there are multiple date fields in the picture data, so you might want to test it small scale to make sure you change the right ones. Might it do what you want?
(Edit: it's a Windows prog in case you are using a different OS)