Date/Time Shift Feature ^
Have you ever forgotten to set the date/time on your digital camera before taking a bunch of pictures? ExifTool has a time shift feature that makes it easy to apply a batch fix to the timestamps of the images (ie. change the "Date Picture Taken" reported by Windows Explorer). Say for example that your camera clock was reset to 2000:01:01 00:00:00 when you put in a new battery at 2005:11:03 10:48:00. Then all of the pictures you took subsequently have timestamps that are wrong by 5 years, 10 months, 2 days, 10 hours and 48 minutes. To fix this, put all of the images in the same directory ("DIR") and run exiftool:
exiftool "-DateTimeOriginal+=5:10:2 10:48:0" DIR
The example above changes only the DateTimeOriginal tag, but any writable date or time tag can be shifted, and multiple tags may be written with a single command line. Commonly, in JPEG images, the DateTimeOriginal, CreateDate and ModifyDate values must all be changed. For convenience, a shortcut tag called AllDates has been defined to represent these three tags. So, for example, if you forgot to set your camera clock back 1 hour at the end of daylight savings time in the fall, you can fix the images with:
exiftool -AllDates-=1 DIR
See Image::ExifTool::Shift.pl for details about the syntax of the time shift string.
Source - http://www.sno.phy.queensu.ca/~phil/exiftool/