Dragging from Photos to Finder sometimes changes date, not always

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 of what the file date says, this is the actual time recorded by the camera.

Photo applications like Photos, CaptureOne, Lightroom, Mylio, 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.

There are apps that will edit the file data to match the Exif. This is one:

https://www.publicspace.net/ABetterFinderAttributes/index.html

When you export the original file (File > Export > Export unmodified original) the original file creation date may be preserved, as long as the file has not been uploaded to iCloud or the library moved between drives, or you are not exporting o a different volume. But any photo taken with a camera or will usually have an embedded EXIF tag with the original capture date, and you can see this date in any program designed to work with photos, for example in Preview > Tools > Inspector. Even the Finder can show you the "content Created" exif tag, if you are browsing the image files in the column view. My Mac is currently set to use the German language - with Englich as the primary language the tag "Inhalt Erstellt" would appear as "Content Created".

It has been always this way. When you are dragging a photo from Photos, you are saving the current preview of the photo. Sometimes the original image file doubles as the current preview (if the photo has no adjustments applied and is small enough to suffice as a preview), but if the photo has been edited or the original is very large, a preview for the media browser will be rendered, and the preview will be an image file with a new creation date.

A new preview file will have a new creation date, while the original may still have the original creation date. But even, if the original image file is used as the preview and a copy of the original dragged to the Desktop, the file creation date may have become changed, for example, if you are syncing your Photos Library with iCloud Photos. Files downloaded from iCloud Photos may be downloaded with a new creation date.

Bear in mind - this means the files you drag out of photos are not a full quality image. They may be lower resolution, they will very likely have higher compression.

You should not use this method for archiving the only copy of your photos. Instead, you should select them, and use file>export, then choose full size and maximum quality.