How to prevent date corruption when fixing metadata in Photos library on Mac?

Stored on my Mac, and not optimized with iCloud, so full images on the mac.

No not stored externally. On my Mac and in iCloud. Mix of jpeg and RAW.

I revised the original query writeup but was unable to upload it before you saw it: I have just begun the process of digitizing selected photos among thousands of my 35mm slides and importing these into Apple Photos library. I am using the Kodak Slide N Scan scanner, which does allow one to input photo dates in saved metadata for each digitized photo. For some inexplicable reason this Kodak scanner only allows one to input photo dates into metadata starting in year 1980 and later. So for all the pre-1980 slides I have to adjust the photo date of imported photos once they have been imported into Photos. It appears that in the process of doing this for the imported photos, dramatic changes were made in the dates of a vast number of previously-existing photos in the library. Now tons of my photos in the library that had been correctly dated are now dated many years later than the correct date, even dated well into the future. This corruption of existing photo dates has never happened to me before this, so I have to believe it is somehow connected to the just-begun importing of digitized photos and adjusting the dates for these. Thankfully I have TimeMachine backups and can restore the Photos library prior to when this all occurred. But I don’t understand how this happened so I can avoid it happening again as I continue to digitize and import more 35mm slides. Would greatly appreciate advice.

Thanks very much. As I think I described in the revised problem description writeup, the Kodak Slide N Scan digitizer inexcusably only permits one to assign photo dates with years 1980 and later, whereas I am digitizing a large number of slides dating back to 1958. Using the Kodak digitizer I was able to date the photos in each batch (a specific set of slides, e.g. Europe trip 1961) with a fairly correct month and day --- I am really not concerned about time of day, but always having to assign the year 1980. So what I was intending to do in Photos with "Adjusting Date and Time," when I must have caused the wide-spread date corruption, was to just shift the photo year of a small batch of photos to the correct year, e.g. change 1980 to 1961, which I would think is applying the same offset to all the photos in the batch. It is quite apparent that in Photos I didn't select the intended batch of photos correctly. I will be much more careful in future. Could it be that what I may have done to cause this widespread date problem in the Photos library was to select all the photos one of the Albums I had just created (e.g. Europe trip 1961, a whittled down set of photos from that trip) and make the date adjustment there, rather than selecting the photos to make the date adjustment out in the Photos Library itself?