Why Do My Photos Say December 31, 1969?
That date means zero. The photo has no date at all, and your computer is showing you the beginning of computer time.
Computers count time in seconds from 1 January 1970. When a photo has no date, the count is zero, and depending on your time zone that shows as 31 December 1969 or 1 January 1970. Your photo is not from the sixties.
It means the date field is empty. Google Takeout exports do this, because Google keeps the date in a separate .json file. Some downloads and conversions do it too.
The real date is almost always recoverable. Takeout JSON Metadata Fixer reads it from the .json beside the photo and writes it back in. It runs on your own computer and is free for your first 500 photos.