Migrating Aperture library to Photos on Sierra

You can correct the date and time of imported photos, if you select them and use the command "Image > Adjust Date & Time". This is described here: Add titles, descriptions and more to photos using Photos on Mac - Apple Support (scroll down to the paragraph: Change a photo’s date and time).

How can I correct the dates of already imported photos?

Could I go in the library structure and do it? Or would that not update the view?

You cannot change the date in the library structure. This is different from Aperture. The photos will only be read once on import, and if you modify the originals later, it will not update the edited versions. Photos is meant to be used with the originals stored in iCloud, where you cannot access them directly anyway.

Is a database rebuild required? How would I do this? Or is there another way?

When you use the commands in Photos to correct the date & Time, there is no repair required.

If you ever need to repair the library, it is done similar to Aperture. (Repair the library)

I have found 3 libraries in my Pictures folder:

• Aperture Library 2.aplibrary (5 GB)

• Aperture Library.aplibrary (8 MB)

• Photos Library.photoslibrary (4 GB)

Could I delete both Aperture libraries after upgrading to Catalina?

You have only one Photos Library , so it seems, that you only converted one Aperture Library to a Photos Library.

If your two Aperture Libraries are not holding the same photos, you will have to migrate the second library as well to Photos. After your libraries have been converted to Photos Libraries, you can delete them from your system volume, but I would keep a copy on an external volume to save your original photos.

This does not work for me either.

Try to select all photos in the Photos view instead:

• Click "Photos " in the sidebar, select one of the photos, then select all with ⌘A.
• Now try to render previews :

1. 1. Choose Edit > Select All to select all of your photos.
   2. Press and hold the Option key, then choose Photos > Generate Previews.
   3. Aperture now generates full-size previews for every photo in your library. To follow its progress, choose Window > Show Activity from the menu bar. Quit Aperture when processing is complete.

This would only change date/time in Photos' metadata database - and not in the originals, correct?

Yes, Photos will only change the versions in the library, not the originals.

If you want to change the metadata of the originals, you have to do it while you still cause Aperture.