Before, with iPhotos it was pretty easy to extract files: just drag and drop and they would have the exact same date and time as when they were imported/taken. Now, with the Photos app, this is no longer the case. WTF. A dropped file gets the current date and time. While I can understand why this is the case, it is not very convenient.

Luckily there is another way to copy files out of your Photos library.

If you visit the Pictures folder, you can see a (in my case very large) file called Photos Library.photoslibrary. If you right click and select Show Package Contents you can browse the individual files within the library. The images can be found in the Masters folder. The files/folders are organized by year/month/day which might not make sense to you, but I find it very useful.

I copied the entire Masters folder to my external drive, and then every image-file retained its original timestamp (yes!).

Now I can free some diskspace without any hesitation :)