I'm looking at implementing this and thought I'd bounce my idea off those interested to get some feedback before starting.
As I see it there should be three parts to this:
1. A script, to be run once, that will go through the database and pull the EXIF date/time from each picture already in the gallery and place that date into the ctime field for that picture.
2. Add a setting to the config to choose whether to use the current/upload date or the EXIF date for ctime when uploading new images. If set to EXIF then add code to pull the EXIF date (if it exists, otherwise the current date will be used) into ctime when adding pictures.
3. Add a field to the edit description to show ctime. This allows you to see what coppermine is using to sort your pictures by and to make changes if there wasn't any EXIF info to use. For example, if I upload some scanned negatives I'd like to be able to set the date/time they were taken rather than using the current time, but there isn't any EXIF info (unless I added it through another program).
Does that make sense? Any suggestions?