NOTE: Due to an Android bug in recent versions (maybe 3.1+), this app is unable to update file timestamps. The bug in question is: http://code.google.com/p/android/issues/detail?id=18624
This is more a script than an application and has no real user interface. Copying images to different folders or SD cards can result in their timestamp getting updated and as a result, the Android Gallery no longer sorts your photos by the date on which they were taken.
This app looks up the EXIF tags of photos and if the timestamp has been saved, replaces the timestamp of the file with this date and time indicating when the photo was actually taken. It operates on one folder at a time and given the integration with Android Gallery to choose the folder, you will have to pick a photo within the intended album to update the timestamp of all photos in that folder.
Only photos with "jpg" extension and an EXIF timestamp will have their files updated with the mentioned date and time.