My photo libray has a bunch of photos showing in timeline with date of 12/31/1969

I’ve seen posting regarding dates not allowed before 1/1/1970 (that is typically been used by PC’s as the “beginning of time”. I have modified all the relevant dates (EXIF is not a WINDOWS date, just a version) to the proper timeframe, yet rescanning the library has not changed anything. On top of this, these show in a strange place - after 8/27/2017 and before 6/17/2017 (and none of the 12/31/69 group are pictures taken in or about 2017). Even modified each photo’s Plex tag (Originally Available ) to the correct date - no help - though this field did actually show 1969-12-31 orginially???