- If you use iPhoto, just use Photos -> Adjust Date and Time (as pointed out by Silus Grok).
- If you use Aperture, follow these instructions.
- If you have neither, you can use PhotoInfo.
I take a lot of pictures, and often when I get home I discover that my camera's clock wasn't set correctly—because of a timezone change while traveling, daylight saving time, dead batteries (and thus a reset clock), or just simple clock skew due to inaccuracy in the quartz crystal and the circuit that drives the clock ticks.
Why timestamps matter: Incorrect timestamps aren't just annoying: they can cause inaccuracy with automatic geotagging (e.g. with GPSPhotoLinker), and they can cause your photos to be in the wrong order when you combine yours with a friend's and sort by time. To combat incorrect timestamps, I usually take a picture of an accurate time source with my camera—either my GPS, or a cell phone. Then, when I get home, I can change the timestamps of all of the pictures on my computer.
No good free tools: Well, that's the theory, at least. I've been looking for years for a free, easy to use tool that will let me change timestamps, but with little success. A Better Finder Attributes will do the trick, but it costs money. I'm cheap, so instead I have used exiv2 for quite a while. It gets the job done, but it's an unfriendly command line tool.
PhotoInfo to the rescue: Happily, today I discovered PhotoInfo by Jim Merkel, which is a Mac OS X application that does almost exactly what I want. It lets you select several pictures (or a folder), enter a time offset in hours:minutes:seconds, preview the changes, and then adjust all of the pictures with one click. It can adjust both EXIF and filesystem timestamps, and it works with JPEG, TIFF, and raw files. Thanks Jim!
The only other feature I'd like to see is for it to automatically calculate the time offset, like iPhoto. I'd like to be able to enter the correct time for one photo, and then adjust all of the other photos by the same amount.
Screenshots of PhotoInfo 2.0.1: