I have a few comments which may be helpful to either of you:
* n3n: to change the EXIF field display from "DateTime Original" to "Photo Added" (or whatever you want), you can change this in the language file, for example lang/english.php for English. It's in the $lang_picinfo array (search for this or for the field you want to change). I would recommend *not* changing the original english.php but instead copying it over to something like english-mod.php, changing that, and setting your language to that. ...unless you let your users change languages. In that case, copy the original file over to a backup directory somewhere else (as english-orig.php) and modify the lang/english.php directly. Then, when doing an update in the future, you can compare the original to the modified one so you can apply your modifications to the new files. Of course if you are letting your users change the language, you'll have to translate "Photo Added" into all the languages.
* it looks like you're trying the "Sort by EXIF date" mod, Cyclist, so maybe that will do what you want (once it's ported by you and/or others for 1.4.2)
* Cyclist: if you find that DateTime Original and DateTime Digitized are the same in another program but not in Coppermine, maybe there are some extra characters in the EXIF info that Coppermine EXIF parser (which is a third-party parser) is having trouble looking past. I found that original photos from my camera work fine in Coppermine (all fields are parsed correctly), but if for example I rotate a photo using Windows XP's Picture Viewer, the photo's EXIF info looks fine in an EXIF viewer (and even in Windows Properties), but Coppermine shows some messed up fields (mostly extra numbers and the height or width fields go screwy). I haven't figured out which characters or more likely the modification of the EXIF structure (since WinXP deletes the Orientation field after rotation) messes things up. If I rotate again, fields change again and are still messed up. I'm ignoring this right now until I figure it out and keeping the raw photos so I have the pristine EXIF info available. (Yes there are utilities to backup the EXIF info but I haven't played with any enough to use it routinely.)
In any case, my guess is that your missing DateTime Original field in Coppermine is due to such a modification to the photo, but I could be wrong.