Dec 31 1969 is a magic number: the lowest date that MySQL will display when it finds invalid data.
So, it begs the question: what is the actual data in your database set to?
You'll need to use phpMyAdmin to look at the date_added field in your products table. There are various tutorials on phpMyAdmin use available online. Maybe even in your hosting company's help system.
If you've been mussing with date display formats or changing locales or non-english languages that you've set your system to try using an invalid date format and it's confused so it's just displaying that magic value.
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