Picture date-time stamp editing

I use Google's Picasa to edit / track my pictures, but I was very disappointed to learn that it does not have a basic tool to edit the date/time stamp of a JPEG.

Simple scenario: Your not-so-technical friend sends you 200 pics taken with their brand new camera. But of course the camera date was set to Jan 01 2001. So now you need to update all the pictures with the correct date in the header.

Solution: Go download a small free command line tool named jhead.
To change the date simply go:
jhead -da NEWDATE-OLDDATE *.JPG

(Looks like their documentation incorrectly specifies the opposite). And try it on a single file first. Their full documentation is here. It calculates the difference between OLDDATE and NEWDATE and applies that difference to all the pictures. So simple and so powerful.

To have Picasa notice the updated JPEG header, you may have to change the date stamp of the file too (not only the JPEG header). To do that:
jhead -ft *.JPG

Nifty.