Is the file creation date 2085 or is the EXIF data 2085? How to find out...
If you know where your images reside, select one in Finder and press Command + I to get the file creation and modification date, then open the image from Finder with Preview and take a look at the Info Panel's EXIF pane and search for the dates in there.
If the Genius Bar guy is right, you need to batch edit just the EXIF metadata. It's different to edit the file creation or modification date or the EXIF timestamps.
Tools for editing EXIF could be
EXIF SYNC or
ExifTool or
pyExifToolGUI or
SetExifData.
I guess you'll need need to export and reimport to Photos to do the modification. But you can try without. In any case, I recommend to make a backup before beginning.
EDIT: As you seem to expect a step-bystep guidance, you'll need to work with us and give us a hint, where exactly we should help.
1. Do you know about the 3 different ways to handle images in Photos (iCloud, local within photos library -> duplicates your files, local at your place of choice -> uses files without duplicates)? Which is your strategy with Photos? Standard storage is within photos library in your User's Pictures folder.
2. Can you locate all affected files and separate them from intact files or do you need help with that? Do you have all the original files somewhere else and did you make changes to your images in Photos app that needs to be preserved?
3. Is the time shift consistent and do you know the time you want to set?
4. I'd recommend ExifTool as I don't know any Photos plug-ins that are more convenient. Are you willing to work with Terminal commands?