Merging Photo libraries and preserving adjusted timestamps

Of course a normal export timestamps everything to today...

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 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.

So, when you refer to the timestamp, what are you referring to, exactly?

Will Aperture actually allow me to export into an existing Photos library?

No.

(I'd previously been using Photos for most happy snaps but Aperture for scanning and editing old photos and slides.)

This will be a problem. Photos 5.0 on Catalina does currently not handle the adjusted capture dates of scans well. It has been much better in the earlier versions of Photos.

For photos taken with a camera the capture date will be embedded in the EXIF tags or IPTC tags. But for scans the EXIF tag with the capture date can be missing, this will depend on your scanner software.

We can add a capture date to our scans in Aperture, as you have done. This will add the date to the edited version but not to the original image file, unless you let Aperture write the metadata into the original image files. Have you done this in Aperture?

Photos will preserve the custom capture date, when you open the library in Photos. And the previous versions of Photos added the capture date to the exported image files as an EXIF date. This is now different in Photos 5.0. Since the Catalina upgrade Photos does no longer embed the capture date into the exported image versions, if the original files are scans and do not have a capture date tag set by the camera. Currently you cannot move edited versions of scanned photos between Photos Libraries and preserve the adjusted capture dates, if the photos are not recognised by Photos as taken by a camera.

More here: Problem in Photos 5.0: The adjusted date is not correctly embedded in the EXIF when exporting from Photos - Apple Community

Before I migrated my Aperture Libraries to other applications, I used the command "Metadata > Write IPTC Metadata to Original" in Aperture. This wrote all titles, descriptions, keywords, locations, capture dates I had added in Aperture into the original image files. It is a very convenient way to save all metadata directly with the originals. My older scans are exported with the correct capture dates embedded. You just have to be sure, that you corrected all typos before writing the metadata to the originals, or the typos will annoy you forever.