it just isn't shown in Mac's Finder (why?????)
Because the Finder doesn't support Exif, which is photographic metadata, only file metadata. The Finder is, after all, a file manager not a photo manager, like Lightroom.
There are two kinds of metadata involved when you consider jpeg or other image file.
One is the file data. This is what the Finder shows. This tells you nothing about the contents of the file, just the File itself.
The problem with File metadata is that it can easily change as the file is moved from place to place or exported, e-mailed, uploaded etc.
Photographs have also got both Exif and IPTC metadata. The date and time that your camera snapped the Photograph is recorded in the Exif metadata. Regardless if what the file date says, this is the actual time recorded by the camera.
Photo applications like Photos, iPhoto, Aperture, Lightroom, Picasa, Photoshop etc get their date and time from the Exif metadata.
When you export from iPhoto to the Finder new file is created containing your Photo (and its Exif). The File date is - quite accurately - reported as the date of Export.
However, the Photo Date doesn't change.
The problem is that the Finder doesn't work with Exif.
So, your photo has the correct date, and so does the file, but they are different things. To sort on the Photo date you'll need to use a photo app.
You can go to the Photo App in finder, right click (control-click) and choose "show contents", then navigate your way through the "Master" folder and find the original file, then drag and drop it to the folder you want it in.
If you're going to give "advice" to people please warn them first when your suggestion will mean dataloss and database corruption. Never ever make any changes within the Library package as it will cause dataloss and corruption. It's also quite unnecessary.
