Thanks for any help - I have tens of thousands of photos with the wrong date on them!
You are looking at the file creation date of the photos in the Finder, right? The fiel creation date will change, when you create a new file as the backup copy. But the embedded EXIF tags in the saved Ile should be showing the correct date. If you open the saved photo in any decent Photo Editor, you should be seeing the correct date.
How have you been creating the backed up Photos? Did you simply drag the photos out of Photos? In that case you will just have saved the preview of your photos and not the full resolution originals. Usually Photos will save the photos with the original file creation date, if you select the photos in Photos and use the command "File Export Export unmodified original". This will save the photos in the same quality as you originally imported them to Photos. But if you have been using iCloud Photos Library the file creation date may even change, if you export the originals.