Here are a few of the times that I have needed to do this: - Changing the battery on my camera switched to a default date. - I wanted to synchronize the time on my camera to a GPS track so the photos matched the timestamped coordinates. - At a family event where images from different cameras were added together.

You can do edit the timestamp using a GUI and many photo manipulation applications like the GIMP support metadata editing. For example on KDE:

gwenview -> plugins -> images -> metadata -> edit EXIF 

The problem is that this gets tiresome after a few images, and anyway the times are correct in relation to each other - I just need to add or subtract a time correction to them en masse.

The answer: exiv2 - Image metadata manipulation tool. It is a program to read and write Exif, IPTC and XMP image metadata and image comments.

user@pc:~$ exiv2 *.jpg
File name       : test.jpg
File size       : 323818 Bytes
MIME type       : image/jpeg
Image size      : 1280 x 960
Camera make     : FUJIFILM
Camera model    : MX-1200
Image timestamp : 2008:12:07 15:12:59
Image number    :
Exposure time   : 1/64 s
Aperture        : F4.5
Exposure bias   : 0 EV
Flash           : Fired
Flash bias      :
Focal length    : 5.8 mm
Subject distance:
ISO speed       : 160
Exposure mode   : Auto
Metering mode   : Multi-segment
Macro mode      :
Image quality   :
Exif Resolution : 1280 x 960
White balance   :
Thumbnail       : image/jpeg, 5950 Bytes
Copyright       :
Exif comment    :

The trick is to pick a image where you can that figure out what the time was and work out the time offset. In my case I needed to adjust the date forward by six months and four days while changing the time back by seven hours. I used the command exiv2 -O 6 -D 4 -a -7 *.jpg

-a time
    Time adjustment in the format [-]HH[:MM[:SS]].
    This option is only used with the 'adjust' action. Examples:
        1 adds one hour,
        1:01 adds one hour and one minute,
        -0:00:30 subtracts 30 seconds.
-Y yrs
    Time adjustment by a positive or negative number of years, for the 'adjust' action.
-O mon
    Time adjustment by a positive or negative number of months, for the 'adjust' action.
-D day
    Time adjustment by a positive or negative number of days, for the 'adjust' action.

When we run this we can see that the timestamp has now changed.

user@pc:~$ exiv2 *.jpg | grep timestamp
Image timestamp : 2009:06:11 08:12:59

That's it. Remember this is the end of the conversation - to give feedback you can either record a show for the HPR network and email it to admin@hackerpublicradio.org or write it on a post-it note and attach it to the windscreen of Dave Yates's car as he's recording his next show.